Fixes #115

Only lookup user configuration before doing a search; implement version completion for Cabal and HLS removal

Setting a version of GHC will fail if provided with a version not installed,
and we don't neede to check the most recent list of GHCs available to know
that.

When running `ghcup set ghc` and pressing tab, one should be able to
autocomplete the currently installed GHCs we have available.

Add an optparse applicative completer for install, rm, and set commands
which shows tags and versions. For installation, all are shown; while
for remove and set, only those installed are.