Add a blhc task

I have a few comments marked with XXX, I'd appreciate some comments on how to proceed with those.

Right now this doesn't support arbitrary options to blhc, just a few known ones. Most likely we don't need arbitrary options, but if we want that to give more flexibility to users, then we should make sure to sanitize that to avoid arbitrary command execution.

Also this only parses input artifacts named as *.build. I'm not sure if we should change that and attempt to parse any text input files, although I don't particularly see the use case.

Part of #168 (closed).