[clang-tidy] Add -extra-arg and -extra-arg-before to clang-tidy-diff.py

Summary:
These flags allow specifying extra arguments to the tool's command
line which don't appear in the compilation database.

Reviewers: alexfh, klimek, bkramer

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D29699

llvm-svn: 294491