Add clang-tidy -header-filter option

Summary:
Add clang-tidy -header-filter option to specify from which headers we
want diagnostics to be printed. By default we don't print diagnostics from
headers. We always print diagnostics from the main file of each translation
unit.

Reviewers: djasper, klimek

Reviewed By: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D3590