Support for getting annotations from docstrings

Hi, me again. I created a tool, called [`doc484`](https://github.com/chadrik/doc484) much like `pyannotate`, for adding type comments based on docstrings. When I created my tool I was aware of `pyannotate` but assumed it just about generating annotations from types captured at runtime. Now that I realize it's really two separate tools, I think there are a lot of great features that could be added to `pyannotate` to make it a one-stop shop for generating/modifying type annotations.

`doc484` is also implemented using `lib2t3` and would be incredibly easy to add to `pyannotate`. It implies some additional features, like the ability to specify a fixer that should be considered a source of truth and should thus overwrite existing annotations if they are out of date (and dealing with the ramifications of that in a multi-fixer scenario).

Let me know what you think.