configure autoprefixer for styles

> Not a blocker, but some of these prefixed properties aren't needed anymore. It would be nice to get [autoprefixer](http://georgeowen.co.uk/blog/autoprefixer-jekyll/) set up to avoid having to manually write / check this.

_Originally posted by @rileyjshaw in https://github.com/SFPC/sfpc.github.io/pull/204_