Add filter to allow selective exclusion of automatic term headlines.

If you want to be a bit more selective of which taxonomies the plugin affects, you can use the `genesis-automatic-term-headline-exclusion` filter. It's a boolean filter, where any truthy value will stop the term headline from appearing.

The logic now comes down to this:

1. If a custom headline has been added, show it.
2. Else if the exclude filter returns true, then show nothing
3. Else show the automatic tern name.
Fix line space in documentation.