Add async execution ability

Readme.md

@@ -64,6 +64,32 @@ module.exports = {
 }
 ```
 
+### Same example with async `.json.js` file
+
+```js
+// data.json.js
+
+var fs = require('fs')
+
+// If module exports a function, it becomes asynchronous.
+// After all hard work it should send results to resolve function
+module.exports = function (resolve) {
+  fs.readFile(__dirname + '/../../Readme.md', 'utf8', function (err, readme) {
+    readme = readme.split('\n')[0] // (just grab header for demo)
+
+    // any other dependencies that are only used in here won't be included in bundle
+    var tape = require('tape') // some random dependency
+
+    // Resulting object should be sent to resolve function
+    resolve({
+      readme: readme,
+      tape: tape, // tape happens to be a function so it won't serialise.
+      random: Math.random(), // will be fixed to whatever value is generated at compile-time
+    })
+  })
+}
+```
+
 ```js
 // index.js
 console.log(require('./data.json.js'))

index.js

@@ -1,9 +1,33 @@
 'use strict'
 
 module.exports = function toJSONLoader (code) {
-  return 'module.exports = ' + JSON.stringify(this.exec(code, this.resourcePath)) + '\n'
+  var result = this.exec(code, this.resourcePath)
+
+  if (typeof result !== 'function') {
+    return toJSON(result)
+  }
+
+  var callback = this.async()
+  if (!callback) {
+    if (result.sync) {
+      if (typeof result.sync === 'function') {
+        return toJSON(result.async())
+      } else {
+        return toJSON(result.async)
+      }
+    }
+    this.emitError('Resource ' + this.resourcePath + ' needs async but it is not supported')
+  }
+
+  result(function asyncCallback (result) {
+    callback(null, toJSON(result))
+  })
 }
 
 module.exports.pitch = function pitch () {
   this.clearDependencies()
 }
+
+function toJSON (result) {
+  return 'module.exports = ' + JSON.stringify(result) + '\n'
+}