labourer => laborer?

Currently, https://github.com/codespell-project/codespell/blob/master/codespell_lib/data/dictionary_en-GB_to_en-US.txt contains:

```
labour->labor
laboured->labored
labours->labors
```

Wouldn't ideally labourer -> laborer and labourers -> laborer also be in there?

I'm happy to add them if you agree.