Support `reveal_type` from dmypy

[rowillia]

Feature

Support invoking reveal_type from dmypy (similar to suggest) to enable tooling to use mypy for type information.

Pitch

libCST is an extremely powerful tool for supporting refactoring Python code, and it has support for type information, but currently only supports Pyre:
https://libcst.readthedocs.io/en/latest/_modules/libcst/metadata/type_inference_provider.html#TypeInferenceProvider

Pyre and Mypy result in slightly different type information so I'd prefer to get our type data from mypy itself.

Existing solutions seem pretty janky -
https://github.com/fortana-co/sublime-mypy-reveal/blob/master/mypy_reveal.py#L72-L102 modifies the code and invokes reveal_type, https://github.com/matangover/mypy-vscode/blob/master/README.md#caveats imports mypy which isn't well supported.