Retrie plugin

[expipiplus1]

It seems like retrie would make a good plugin. TODO, figure out exactly what functionality would be appropriate to expose.

Perhaps this would be better placed in the https://github.com/haskell/haskell-language-server, I assume there'll be some issue migration at some point?

[jneira]

Looks really good. I guess the new plugins would be added directly to hls, once it has the base work to to do it: haskell/haskell-language-server#45

[alanz]

The plugin support landed via that PR, it is now in master. The concept is very similar to that in hie, but the process of getting information from GHC is slightly different. But you can see how it works from e.g. the ormolu plugin, which uses the GHC AST