Allow for passing Rouge formatter as a String

[kejadlen]

I thought this might be handy so that Jekyll users can change the Rouge formatter.

[gettalong]

Thanks for your pull request but I'm not really convinced that this is a good change... will have to think about it.

[kejadlen]

Yeah, I can see that - do you suppose the String to Class transform might make more sense in Jekyll than Kramdown? I'm not too familiar with either project, so don't know what's reasonable.

[gettalong]

@kejadlen Thanks for your pull request! After some consideration, I have changed your pull request a bit (remove unnecessary changes, changed String to a more restricted regex) and included it.

I.e. this will be in the next release.

[kejadlen]

Cool, thanks!