Consider supporting dmypy

The latest version of mypy now ships with its own special daemon for checking for type errors faster. It might be nice if ALE supported this in some way or another, possibly including an additional plugin which uses ALE.

## Implementation Notes

`dmypy` writes a `dmypy.json` file which looks like this:

```json
{"pid": 26666, "sockname": "/where/ever/dmypy.sock"}
```

`dmypy` doesn't return accurate results yet, just approximate ones.