Support of the BitBake language

This pull request was asked for in #1074. It adds support for the BitBake language.

It creates a conflict on the .bb extension with BlitzBasic.
I tested it on several BitBake repositories (which are currently recognized as BlitzBasic) and I didn't have a single perfect detection. I think this is because they aren't that different and, on the small files, the Bayesian classifier isn't very helpful. (It's better than before as BitBake files were not detected at all.)
@e8johan Do you have more representative BitBake files? It could help improve the accuracy of the Bayesian classifier.

I think it could be resolved with heuristics as discussed in #1074. I could write these when heuristics are enabled.

Because of the current restriction, I had to change the primary extension of BlitzBasic to .decls.