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.
The wording on this ("...makes RSpec first match the default pattern...") makes it sound like `--exclude-pattern` can't be combined with `--pattern`.  If that's true, we should find a way to make them work together. If it's not true, we should find a better wording for this :).