Adding Bleach callback support

I had the need to use a Bleach callback to specifically not link text such as `setup.py`. This comes up a lot for me, since I'm using this to parse what are usually `README.md` files.

I noticed mdx_linkify didn't give any ability to use this feature of bleach, so I added it using the standard config setup defined in the Markdown API.

I added a test which is essentially my own use-case for the feature. It works, but it might not be the most exhaustive.

All my additions are Python 2.7 and 3.4rc3 tested and `pep8` approved.