[clang-tidy] Add support for custom configuration file names/formats.

Summary: We're using different clang-tidy frontends (command-line, batch analysis jobs, code review integration), some of which are limited in the choice of configuration format. In order to avoid duplication of configuration information, we need to support the same configuration format in the command-line tool. This patch adds an extension point to make this possible without rewriting FileOptionsProvider.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5821