Add an hook into `hie.yaml` config such users can specify to use cabal-helper

[fendor]

Currently, the only real supported cradle types are stack and cabal (maybe direct, too). However, if there is no hie.yaml, we default to cabal-helper. It should also be possible to always select cabal-helper.
Issue needs to be addressed in hie-bios.

[alanz]

And perhaps we should add this info to the readme?

[fendor]

What information in particular?

[alanz]

That in the absence of a hie.yaml we use cabal-helper to determine the settings.