[clang-tidy] support --load in clang-tidy-diff.py/run-clang-tidy.py

Support for loading shared objects as plugins into clang-tidy was added
in http://reviews.llvm.org/D111100. Unfortunately, the utility scripts
`clang-tidy-diff.py` and `run-clang-tidy.py` did not receive
corresponding arguments to forward such plugins to clang-tidy.
This diff adds a `-load=plugin` option to both scripts.

Differential Revision: http://reviews.llvm.org/D12306

Reviewed By: aaron.ballman