Implement textDocument/references

[Delapouite]

Hi

I'm starting this issue to keep track of the current status of things in regards of supporting textDocument/references in the future.

This project is heavily base on the nuclide integration of flow. Work is under way to provide an efficient find-refs over a project:

• https://github.com/facebook/nuclide/issues/688#issuecomment-389247593
• https://github.com/facebook/flow/releases/tag/v0.73.0