Add --exclude_pattern option to configuration

Created by: jmondo

Resolves #1626 (closed). 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.