Include separate export for transformStyles method

[adamgruber]

It may be useful for some to be able to use this package outside the confines of a plugin. Exporting/bundling the transformStyles method separately would allow users to include it like this:

import transformStyles from 'sass-extract-js/transformStyles'

[adamgruber]

As of 0.2.0, the tranformStyles method has been renamed and can be imported and used directly outside of the sass-extract plugin pipeline.

import transformVars from 'sass-extract-js/lib/transformVars'