Replace Redshift option with generalised option for multiple extra keyword types

[darkixion]

At the moment, there is a specific option for adding Redshift keywords, but I think this may perhaps become more flexible if it were generalised. For example, EDB Advanced Server uses some syntax not supported by community Postgres, so (just as an example), it would be nice to have:

-K redshift or -K epas

Perhaps even allow specification of a custom keywords file (am I over-engineering here?), where you could specify:

--extra-keywords timescaledb.keywords