Provide flow typings

[mweststrate]

MobX ships with typescript typings. However, flow people are regularly asking for these typings as well. It would be nice to add these typings as well.

Some questions:

1. can the conversion be automated? see e.g. https://github.com/Kegsay/ProbablyFlowTyped
2. can the typings be maintained in this repo, or do they need to be stored in the flow typings registry?
3. what about mobx-react / mobx-util?

PR's are welcome!

[A-gambit]

@mweststrate I need it in my project. I can do it the next time.

[mweststrate]

Awesome, looking forward!

[mweststrate]

@A-gambit check this: https://github.com/joarwilk/typedef-converter. Might be useful!

[A-gambit]

@mweststrate thank you. I have started writing types locally, but I had a problem with function arguments. For example: https://github.com/mobxjs/mobx/blob/master/src/api/observe.ts#L12-L17. I think this library will be useful, thank you.

[A-gambit]

Already done in MobX3!