Support for opam-based environments

[hcarty]

When using opam without esy, #3 allows vim to start editing Reason or OCaml without immediately throwing up errors. Unfortunately none of the :MerlinTypeOf and similar commands are available for me when editing Reason code when using:

• opam
• merlin and reason pinned to --dev-repo
• This plugin + the vim plugin that comes with Merlin

Automatic error highlighting on save does work and is wonderful. I'm using ALE if that makes a difference.

[hcarty]

I suspect there's a detection issue somewhere. If I have a *.re file open and manually run

:let res = merlin#Register()

then I have access to :MerlinTypeOf and friends.