[clang-format] Add option to explicitly specify a config file

This diff extends the -style=file option to allow a config file to be specified explicitly. This is useful (for instance) when adding IDE commands to reformat code to a personal style.

Usage: `clang-format -style=file:<path/to/config/file> ...`

Reviewed By: HazardyKnusperkeks, curdeius, MyDeveloperDay, zwliew

Differential Revision: https://reviews.llvm.org/D72326