Add support for -X/--exclude/EXCLUDE_FILES.

RBTools now recognizes the -X and --exclude command line options for
excluding files from diffs and review requests. The EXCLUDE_FILES
option in .reviewboardrc adds the same functionality, but can be
committed to the repository.

Currently no backends support this option.

Testing Done:
Unit tests pass.
Running `rbt diff` with `-X` or `--exclude` flags, or with
`EXCLUDE_FILES` set in `.reviewboardrc` causes the expected error
message

Reviewed at https://reviews.reviewboard.org/r/6317/