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.
This wording suggests RSpec will load every file on the file system that does not match the specified pattern.  Maybe  this is better?

``` ruby
# Excludes files that match the specified pattern from being loaded by RSpec.
```