Extract inlined styles

Fixes #2 
Yes, i thought about something like this: https://github.com/yeoman/grunt-filerev/blob/master/tasks/filerev.js#L46. Of course the filename used in https://github.com/bezoerb/inline-critical/blob/master/index.js#L32 and https://github.com/bezoerb/inline-critical/blob/master/index.js#L52 need to be changed as well.
I think this is the only way to avoid conflicts. If you've got another idea please let me know.
The rest looks good to me.