Add RBS feature line

It would be amazing to have a `RBS` feature line output option.

Currently I guesstimate it from `rbs_spacer` and `rbs_motif` but pyrodigal should know where it is on the genome, and maybe even have a score for it.

INDSC deprecated the `RBS` feature key, but the correct way is this:

```
regulatory
/regulatory_class=”ribosome_binding_site”
```

https://www.insdc.org/submitting-standards/controlled-vocabulary-regulatoryclass/