Feature Request: Make variable annotations optional

[saxbophone]

I think it might be a good idea to make the type-checking of variable declarations optional, by passing an appropriate flag to mypy when running static code checking.

The reason I think this would be useful would be for those who might want to have type-checking of function signatures and the calling of them, but not for declaration of variables.

If the maintainers think this is a feature they'd like to have, some further discussion probably needs to happen regarding how the behaviour is handled (i.e. variable checking being optional versus ignored).

This is following on from observations related to #1164

[gvanrossum]

I'm not sure there'd be a huge demand for this. In a sense variable annotations are already optional -- except for cases where mypy can't infer the type (mostly for empty containers). If you want signature annotations but don't care about the body being type-checked period, perhaps you should use stub files, which are exactly that.

[saxbophone]

Aha! Great, thanks @gvanrossum, that looks perfect!