Support for html attributes

[egze]

Hi,

We stumbled upon an issue in our app where the html generated by RedCloth needs to be SEO friendly. For example we need the rel="nofollow" or target="_blank".

Do you see any way how we can have it without using raw html? Maybe we can add some generic syntax for html attributes? I'll be happy to try a PR if you point me to the right direction.

Something like:

"!rel: 'nofollow'; target: '_blank'!RedCloth":https://github.com/jgarber/redcloth/issues/new

Would output:

<a rel="nofollow" target="_blank" href="https://github.com/jgarber/redcloth/issues/new">RedCloth</a>