Letting the user input arbitrary Text in default-extensions

[freuk]

Hi, first of all thanks for working on this openly, it's useful :)

Cabal passes extensions that are not in its KnownExtensions type onwards with a simple warning:

Warning: Unknown extensions: DerivingVia

It would be convenient to be able to allow arbitrary text values for extensions at the Dhall level, additionally to the types generated from KnownExtensions in Extension.dhall by the "meta" code.
Very low priority I guess, since the workaround ({-# LANGUAGE <ext> #-}) is straightforward.

[ocharles]

This seems perfectly reasonable. I'd be open to a Other : Text alternative, and would take a PR adding that. It's unlikely I myself will have time to implement this any time soon, though.

[freuk]

Great - will try my hand at this one.