To make this possible:

	import GridiconExternal from 'gridicons/external';

we need to publish the individual icons in the root of our npm package.

This also changes how do we tell npm which files to include from
whitelisting (using the files prop in package.json)
to blacklisting (using the .npmignore file). The reason for that
is that I've tried to whitelist every individual icon by setting
the files prop to "*.js" and then blacklist only the Gruntfile.js
within .npmignore. It seems that in case of conflict the files prop
takes precedence over .npmignore, so the Gruntfile.js was still included.

Note that this is necessary to prevent the build/ folder
to be included in the npm package.

For some reason, although the build/ folder was blacklisted
in the .npmignore, it was not excluded from the package.
https://docs.npmjs.com/misc/developers#keeping-files-out-of-your-package
The docs don't mention the build/ folder as one of the always
included directories, though.

By using a function instead of React.PureComponent
the transpiled icon is 1,5K smaller. Most individual components
are between 4K and 5K, so this change means a ~30% size reduction.

The idea is that we want to prepare Gridicons to be able
to distribute both CommonJS and ECMAScript Modules.

Initially, the idea was to use the directory root to publish
the CJS modules so others could:

	import external from 'gridicons/external';

The main disadvantage of that approach is that, in the future,
when ECMAScript modules become more common that namespaced
will be taken by the CommonJS modules and we won't be able
to change that if we want to be backwards-compatible.

So, instead, the approach we're taking is being agnostic about
what module system the library user wants to use by default. So,
either:

	import external from 'gridicons/cjs/external';
	import external from 'gridicons/esm/external';

will work.

By creating the files dynamically form a template literal,
we're able to centralize the declaration of icons that need offset.

The example file imports './index.js', so we need that extension
to be used by both the CommonJS and ES modules.

Props to @ockham for the push of publishing both.

In previous commits, I bumped to 3.0.1-alpha and 3.0.2-alpha
instead of 3.0.0-alpha.1 and 3.0.0-alpha.2. I'm sorry!

Taking advantage of the npm unpublish feature I've reverted
those wrong alpha publications so I hope that, when you read this,
they're available to you.