Allow Uglifier options in assets.yml

[bruno-c]

I've added a small amount of code + test to allow passing options to Uglifier.

In assets.yaml:

options:
  max_line_length: 800
  beautify_options:
    indent_level: 4

options is a bit generic for the key but since the CSS minifier has no options to speak of, I thought it was the right name to use.

The only other bit of code is a recursive function that takes the options hash and converts string keys to symbols (as expected by Uglifier). Entering the symbols in the YAML is awkward, so this seemed to be a better option.

Thanks for Evax. It's the simplest thing that could possibly work when it comes to minifying / concatenation.