Refactor to use FontCustom

I extracted the existing glyphs using [a fontforge python script](https://gist.github.com/jvatic/9f193f3579a525416816) and setup [FontCustom](https://github.com/FontCustom/fontcustom).

This makes adding and manipulating icons much easier, but the extracted glyphs appear to be a bit off. Using the original vectors instead of the extracted ones may fix the issue, or some work will need to be done to correct the rendering.

Another way of doing this would be to write a script to modify the existing fonts given an input svg ([the FontCustom generate script](https://github.com/FontCustom/fontcustom/blob/master/lib/fontcustom/scripts/generate.py) would be a good place to start in this case).

Related: #65