Add prepublish script (#942)

* 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

scripts/utils.js

@@ -0,0 +1,14 @@
+module.exports = {
+    /**
+     * Converts a brand title into a filename (not a full path)
+     * @param {String} title The title to convert
+     */
+    titleToFilename: title => (
+        title.toLowerCase()
+            .replace(/\+/g, "plus")
+            .replace(/^\./, "dot-")
+            .replace(/\.$/, "-dot")
+            .replace(/\./g, "-dot-")
+            .replace(/[ !’]/g, "")
+    )
+}