Make the stub generator official

[JukkaL]

The stub generator is pretty useful, but it's undocumented and a little rough around the edges.

Some possible things to do:

• Add script such as mypy-stubgen that is installed automatically and has help etc.
• Document the stub generator.
• Maybe even give a pointer to the stub generator when mypy can't find a module (Better message when we can't find a module #952).
• Do more testing and polishing.
• Add support for generating stubs for Python 2 C extension modules.

We should create separate issues at least for the bigger tasks here.

The stub generator can be currently run as python3 -m mypy.stubgen.

[ilevkivskyi]

OK, it is already official.