Refine options Flow type

[reyronald]

Using Flow's Object types is actually a violation of the unclear-type linting rule:

unclear-type

Triggers when you use any, Object, or Function as type annotations. These types are unsafe.

So, using it to type the exported options object may cause issues for developers that have Flow linting enabled. Also, specifically typing this will result in a better developer experience anyway :)

As I started typing it, I ended up having to use Function and any elsewhere. Ideally those types should be refined too, but since I'm not familiar with Preact's internals I decided to leave it as-is and maybe a reviewer can suggest the right direction?

[coveralls]

Coverage remained the same at 100.0% when pulling 39abf38 on reyronald:refine-options-flow-type into 30c0fd7 on developit:master.

[marvinhagemeister]

Awesome, really appreciate improvements to the FLOW experience. Thanks for making a great PR 👍