WIP: mypy plugin

[sobolevn]

Here's a very quick and very dirty implementation of a plugin that does one thing: reveals the type of glom(person, 'age'). Which is int.

There are also several mypy issues that need to be fixed / ignored.

Reproduction: mypy ex.py

I am sorry, but I don't have much time to work on this further (there are a lot of my own projects that do require a lot of effort from me). So, I would love to pass it on to someone, and of course, I would love to help / review / answer any questions about developing a mypy plugin.

Related to #148

[sobolevn]

Quick update: I have released AttrOf type that does exactly the same thing. Docs: https://github.com/wemake-services/mypy-extras/