`output_style` option

Use this to specify a custom output_style
(http://sass-lang.com/documentation/file.SASS_REFERENCE.html#output_style).

This will default, as before, to using the `nested` output style.  This allows
users to specify a custom output style (ie `compressed` or whatever).
Merge pull request #4 from redding/kr-style

`output_style` option