Add support for .clang_complete files

This allows users to set per-project include paths/defines etc. by providing
a .clang_complete file at the root of their project. The format is
described in the README, but it's one flag per line.

Test-Information:
Tested completion in dev mode on a project that required both custom
includes and custom defines. Completions seem correct.
Merge branch 'Kev-dot_clang_complete' #2