Generate good (but non-noisy) auto PRs

[chenglou]

chenglou/react-motion#366

Our build system is opinionated enough to be able to give you CI support (?), a test infrastructure, and the rest, in such way that when we upgrade something we can send to your github repo an automatic PR (that passes CI and the tests beforehand? So that it's not noisy).

This is inspired by

• The PR above PR.
• Facebook codebase's approach of testing master releases of even e.g. React, whose contributors codemod the entire codebase so that no one gets left behind.
• iOS' auto upgrade codemod when new APIs come out.

Let's be the first language (not library, not platform) to achieve this!