Enabled :minify argument to supply hash to uglifier

Credit goes to @rapind for this one. :+1: 

I needed to customize the options being supplied to Uglifier. The pull request these changes were inspired is here: https://github.com/pferdefleisch/guard-sprockets/pull/12.

I think it would be good to rename the `minify` argument to `js_minify` at some point to be clear that it is only for JavaScript and not CSS.