Extensions to snippets

This is a bit of an ideas issue, let me know if this is inappropriate...

We could support (`^` for cursor location):
1. Autocomplete patterns while being written: `foo Ba^ => foo Bar {..}^`
2. When adding a top level definition: `foo ^ => foo :: _\nfoo = _^`
3. In deriving clauses for typeclasses that exist in `Prelude` (or perhaps even in this module or project? Such as ToJSON/FromJSON)
4. Cabal files could definitely have a lot of autocomplete when adding new subheadings
5. `instance Show Foo where^ => instance Show Foo where\n showsPrec Foo {..} = _^`