Add a step to clean up SVG files before compiling the font

[voidgraphics]

First we must run the svg through this to convert strokes to outlines, then through SVGO to clean everything up. The default settings as seen on SVGOMG should be fine.

This will ensure that the SVGs are properly formatted for use with fontello, and also reduce the file sizes all around.

The cleaned SVG files should probably not replace the source ones, but instead be stored in the storage directory or something, so that the files do not go through the optimisation process more than once, which could result in unwanted side effects.