Support configuring an envFile for the mypy environment

[danlamanna]

Fixes #25, see #25 (comment) for an example to reproduce the issue of needing environment variables. This is most commonly necessary when using the django-stubs plugin because a full fledged django environment is necessary to execute type checking. Warning: I don't really write typescript, I just sort of threw this together and it appears to work for me.

[matangover]

Looks great! Thanks

[danlamanna]

Hey, would you mind merging this and cutting a release?

[matangover]

Hope to get to it in the next few days.
Btw why did you choose env file as opposed to setting it directly in the settings?

[danlamanna]

Btw why did you choose env file as opposed to setting it directly in the settings?

I'm not sure what you mean, why use an env file instead of setting key/value pairs in the settings? I wanted to mimic how vscode-python does things as much as possible (see https://github.com/microsoft/vscode-python/blob/5e5a7ce7b6b516d4919d72a27dc18c9ec3992861/src/client/common/variables/environment.ts#L125).