Add BitBake lexer

[Rahix]

Add a new lexer for BitBake recipes, appends, and classes. Bitbake recipes contain snippets of python and shell scripts so I had to use the delegation mechanism. I wasn't entirely sure, however, how to properly detect the end of such blocks so the mechanism I use now is not 100% faithful of what bitbake does. For most recipes it should work fine, though.

Additionally, BitBake also uses the .inc extension for recipe includes but that is already used by the PHP lexer. I guess that means there isn't any way to nicely allow them to coexist? (bitbake doesn't have any shebang for detection :/)

[MofX]

Ping for review, this would really be nice-to-have