SVG omptimisation

Finally got around to adding svg optimisation to our icon/sprite processing. Uses the node module [SVGo](https://github.com/svg/svgo) to strip all svg files of cruft e.g. excess comments, elements and attributes.

This commit also refactors the spricon output from less to sass.

## tl;dr
Saves **56kb** across all the svg files, which in-turn shaves **74kb** off our main.css file (246 -> 172) pre gzip. Which is a massive saving and very worthwhile for 30mins of work!