Add en_US language to `wp core language` and  	add functionality to filter list of languages

Fixes #1404 
> Is there a way to check that a table has only the expected rows instead of checking that a table contains the expected rows?

Nope. You can just do `wp core language list --status=active --format=csv | wc -l`

> Should we create it?

Eh, if you'd like. I don't think it's necessary