Add hamdown support

[inem]

I am working on new format called hamdown, and would like to add its support to tilt.

[judofyr]

These days I would prefer if you provide the template class in the Hamdown gem itself. Either by depending on Tilt and use the template class as the public API for rendering, or by having a hamdown/tilt.rb file which defines it.

[jeremyevans]

hamdown looks dead upstream (no commits in 3 years), so I think this can be closed.