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.
I'll agree this is a slightly poor spec. Though, it is testing something. It's just not explicitly clear from the spec name. This is likely a duplicate of the spec on [line 408](https://github.com/jmondo/rspec-core/blob/exclude-pattern/spec/rspec/core/configuration_spec.rb#L408-L411). The name of spec 408 is clearer, however, it never sets a `pattern`, so it's arguably incomplete - unless it was intended to by-pass the setting of a default pattern?

Once we sort out if these are dupes, or should be testing slightly different things, I think it's possible to improve the description to clear up any confusion.