Adds plugin support for CoffeeScript 2.x. (#745)

- modifies Webpack config to add appropriate loaders
- adds '.coffee', '.cjsx' to resolvable extensions
- transpiles source before Babylon parsing

packages/gatsby-plugin-coffeescript/.gitignore

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+node_modules
+coffee
+/gatsby-node.js
+/index.js

packages/gatsby-plugin-coffeescript/.npmignore

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+# Logs
+logs
+*.log
+
+# Runtime data
+pids
+*.pid
+*.seed
+
+# Directory for instrumented libs generated by jscoverage/JSCover
+lib-cov
+
+# Coverage directory used by tools like istanbul
+coverage
+
+# Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-task-files)
+.grunt
+
+# node-waf configuration
+.lock-wscript
+
+# Compiled binary addons (http://nodejs.org/api/addons.html)
+build/Release
+
+# Dependency directory
+# https://www.npmjs.org/doc/misc/npm-faq.html#should-i-check-my-node_modules-folder-into-git
+node_modules
+*.un~
+yarn.lock
+src
+flow-typed
+coverage
+decls
+examples

packages/gatsby-plugin-coffeescript/README.md

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+# gatsby-plugin-coffeescript
+Provides drop-in support for CoffeeScript and CJSX.
+
+## Install
+`yarn add gatsby-plugin-coffeescript`
+
+## How to use
+1. Include the plugin in your `gatsby-config.js` file.
+2. Write your components in CJSX or CoffeeScript.
+
+```javascript
+// in gatsby-config.js
+plugins: [
+  // no configuration
+  `gatsby-plugin-coffeescript`,
+  // custom configuration
+  {
+    resolve: `gatsby-plugin-coffeescript`,
+    // options are passed directly to the compiler
+    options: {}
+  }
+]
+```
+
+## Notes
+First, note that CoffeeScript + React is a troubled combination. This plugin
+relies upon deprecated modules that may someday prove to be dysfunctional or
+otherwise deficient.
+
+Furthermore, note that the installed version of CoffeeScript is @next. This is 
+not optional - *named exports are required for page queries to work properly.*
+
+You will need to manually edit your `coffee-loader` installation and install 
+`coffeescript` separately in your project directory to ensure that 
+CoffeeScript@next is being loaded. The very first line of source in the 
+former's `index.js` should be the following: note the lack of dash.
+
+```js
+var coffee = require("coffeescript");
+```

packages/gatsby-plugin-coffeescript/package.json

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+{
+  "name": "gatsby-plugin-coffeescript",
+  "description": "Adds CoffeeScript support for Gatsby layouts and pages.",
+  "version": "1.0.0-alpha12",
+  "author": "Kyle Mathews <mathews.kyle@gmail.com>",
+  "contributors": [
+    "Noah Lange <noahrlange@gmail.com>"
+  ],
+  "dependencies": {
+    "coffee-react-transform": "^5.0.0",
+    "coffeescript": "next"
+  },
+  "devDependencies": {
+    "babel-cli": "^6.22.2"
+  },
+  "keywords": [
+    "gatsby",
+    "coffeescript"
+  ],
+  "license": "MIT",
+  "main": "index.js",
+  "readme": "README.md",
+  "scripts": {
+    "build": "babel src --out-dir .",
+    "watch": "babel -w src --out-dir ."
+  }
+}

packages/gatsby-plugin-coffeescript/src/gatsby-node.js

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+import transform from 'coffee-react-transform'
+import { compile } from 'coffeescript'
+
+const COFFEE = /\.coffee$/
+const CJSX = /\.cjsx$/
+
+export function resolvableExtensions() {
+  return [ `.coffee`, `.cjsx` ]
+}
+
+export function modifyWebpackConfig(ctx) {
+  // we need to use Babel to get around the ES6 export issue
+  const { config } = ctx.args
+  config.loader('coffee', {
+    test: COFFEE,
+    loaders: [ 'babel', 'coffee' ]
+  })
+  config.loader('cjsx', {
+    test: CJSX,
+    loaders: [ 'babel', 'coffee', 'cjsx' ]
+  })
+}
+
+export function preprocessSource(ctx) {
+  const { args: { filename, contents }, pluginOptions } = ctx
+  // don't need to account for ES6, Babylon can parse it
+  if (CJSX.test(filename)) {
+    return compile(transform(contents), pluginOptions)
+  } else if (COFFEE.test(filename)) {
+    return compile(contents, pluginOptions)
+  } else return null
+}

packages/gatsby-plugin-coffeescript/src/index.js

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+// noop

packages/gatsby-plugin-coffeescript/yarn.lock

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+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+
+coffee-react-transform@^5.0.0:
+  version "5.0.0"
+  resolved "https://registry.yarnpkg.com/coffee-react-transform/-/coffee-react-transform-5.0.0.tgz#b62e8ae1b113fce9b1a5990b06c9bc44651ae9c5"
+
+coffeescript@next:
+  version "2.0.0-alpha1"
+  resolved "https://registry.yarnpkg.com/coffeescript/-/coffeescript-2.0.0-alpha1.tgz#abf8b54b5c20a9c9f707c8aff35b227d1d6b5e3b"
+  dependencies:
+    marked "~0.3.6"
+
+marked@~0.3.6:
+  version "0.3.6"
+  resolved "https://registry.yarnpkg.com/marked/-/marked-0.3.6.tgz#b2c6c618fccece4ef86c4fc6cb8a7cbf5aeda8d7"