Allow defining alternative shells

[nc6]

Often in haskell we want to define multiple shells, reflecting things like GHC versions and profiling.

nix develop .#ghc810
nix develop .#ghc96-profiling

Currently Organist doesn't support defining these since the shells attribute is a fixed set.

[thufschmitt]

It just occurred to me now: you can actually do that by setting flake.devShells directly:

flake.devShells.foo = organist.shells.Bash,

It's not perfect, but it works