Preprocessor commands completion

[Yanpas]

It would be nice to have a completion for define, include, if, ifdef, elif, else, endif, undef, line, error and pragma

[Yanpas]

I think it should be implemented as some heuristics before calling clang_codeCompleteAt (clang_complete.cc:524).

Proposed heuristics: If line follows this regex \s*#\s*\w{0,10}