* Add prepublish script
Compiles icons into both static js files (icons/{filename}.js) and
into a static index.js. The latter is not tree-shakeable.
Closes#941, closes#751
* Add information on tree-shaking to README
* Remove index.js after publishing
* Change 'prepublish' to 'prepublishOnly'
* Link to webpack in README
* Simplify postpublish cleanup
Thanks @ericcornelissen
* Fix quote usage in prepublish.js
* Fix quote usage in utils.js
* Optimized file size of prepublish output
* Improved readability of prepublish script
