Add --exclude_pattern option to configuration

Resolves #1626. It allows passing `--exclude_pattern` to `rspec` on the command line.

Afaik we still can't use the power of rake tasks. This is because the rake task still uses `FileList` to include files. And once files are included, they can't be changed by passing `--pattern` or `--exclude-pattern` (there is a warning about this when you try). Fortunately, once #1589 is merged, it will be possible to pass  `--exclude-pattern` within `rspec_opts` in the rake task config.  This is because the rake task's `pattern` option will use `--pattern` which can be combined with `--exclude-pattern` for a one-two punch.

@myronmarston Let me know how this looks! Sorry it took so long.
It is a weird spec, but I think there's value in having a spec that shows what happens when users pass a specific file name (rather than a directory), and how that relates to `pattern`.  It looks like the behavior is that the `pattern` is ignored and the file is loaded since it was explicitly specified.  I think it would be useful to replace this with a spec that calls `assign_files_or_directories_to_run` with two file names -- one that matches the pattern and one that doesn't -- and have it show that both files are included in `files_to_run`, demonstrating that the `pattern` is not applied to specific file names.

This makes me realize we should probably also spec how `exclude_pattern` relates to specific files...seems like it is most consistent to do the same thing: specific files are always loaded regardless of whether or not they match either `pattern` or `exclude_pattern`.

Does that make sense?