Allow per-file clang-tidy options.

Summary:
This patch makes it possible for clang-tidy clients to provide
different options for different translation units. The option, which doesn't
make sense to be file-dependent, was moved to a separate ClangTidyGlobalOptions
struct. Added parsing of ClangTidyOptions.

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3979