Render AsciiDoc files (BB-1913)

Hello,

would be nice if you could render AsciiDoc files (after all it's written in Python).

File extensions to catch:

.ad, .adoc and .asciidoc

(and probably .ad.txt, .adoc.txt and .asciidoc.txt:
https://code.google.com/p/asciidoc/issues/detail?id=13 )

Repo with samples: https://bitbucket.org/matrixik/asciidoc-samples