Simplify.js plugin

Reduce file size drastically on complex polylines. Especially useful with maps.

Supports simplifying `<polygon>`, `<polyline>` point series, as well as simplifying sequential line instructions in path syntax on `<path>`, `<glyph>`, `<animate>`, and `<animateMotion>`.

Uses the Simplify.js library. Demo: http://mourner.github.io/simplify-js/

cc svg/svgo/issues/240 & svg/svgo/issues/9
cc @cheeaun