add vdb debugger backend.

Add a debugger backend for vdb (https://github.com/vivisect/vivisect). The major work was really only figuring out how to get the dbgentry working. It took a few hacks, but seems to work fairly stably. Added a custom lexer because 1) regexes are write-once/never read again, and I was having a bit of trouble reversing the LLDBIntelLexer patterns, and 2) the vdb Intel listing has a column for raw hex bytes.