Fix duplicate CSS with autoprefixer

Fixes #108

> Due to the way the usemin block was set up, it would concat and minify both the
> non-prefixed and the prefixed CSS file. Now, depending on whether you enable
> autoprefixer or not, the usemin search path is adjusted and the snippet is
> programmatically inserted into the index.html template.

/cc @stephenplusplus @addyosmani Could you take a look that I did not introduce another silly mistake?