Register AsciiDoc filter, handled by Tilt

As of Tilt 1.3.4, Tilt now supports AsciiDoc. Register an AsciiDoc filter in Haml that delegates to Tilt.

The first commit includes the register command. The second commit is a test case, which requires adding asciidoctor as a development dependency. I'm only concerned about seeing the test merged if you are...though I do think it's a good idea.