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 think that's clearer. Won't match the wording of `pattern` though. Is that ok?
```
Load files matching this pattern (default: `'**/*_spec.rb'`))
```