Allow passing a custom mypy executable

[person142]

This is one approach to #19.

We have a custom py_binary wrapping mypy that bundles some plugins. Is the project open to allowing users to pass their own mypy? The simple way would be making _mypy_cli public:

https://github.com/thundergolfer/bazel-mypy-integration/blob/master/mypy.bzl#L23

The more proper way would probably be making a mypy toolchain.

Willing to PR this if it's acceptable.

[thundergolfer]

I thinking being able to pass the MyPy binary is entirely reasonable. #30 seems like something worth looking at too.

[adzenith]

This feature just landed in #98