add 'classname' option

here's another one I needed to support my project. my use-case is pretty straight-forward: i want to style detected links differently than others.

the new `classname` option:
* accepts a string w/ length > 0
* uses value as prefix for css classname
* applies classname suffix based on link type (email, twitter, etc)

the thing I don't love as it stands is: the option name doesn't do a great job of explaining what will happen (i imagine most will assume it simply adds the given classname(s) to the anchor tag where it really treats the provided string as a prefix - where `prefix-<link type>` is the result) :stuck_out_tongue_winking_eye:

also, i owe docs for it... happy to tackle that if it seems like a worthy addition. let me know what ya think!