Defining version 4 of react-datetime

We've just released version 3 and we need to start thinking where we are going to with the next version of the library. I'd like to give the opportunity to anyone to collaborate with their code but also with their thoughts about how to make react-datetime better.

React-datetime is an old pal, it was created soon after React.js was published (can you remember React's mixins? react-datetime used them!) and that means that the library's base code was based in old patterns that, nowadays, no developer want to dive in.

So version 3 has been an effort to take react-datetime a bit to the present (we stopped using `createClass`) but the internals are still basically the same, trying to not to break the current API so much.

My view of version 4 is in the same direction, I really would like to:

- [ ] Port the library to typescript
- [ ] Update dev dependencies and dev tools
- [ ] Port the tests to react-testing-library and stop using snapshot testing

But also I would like to make the code better with features that has been requested for so long:
- [ ] Reduce the usage of components' state as much as possible
- [ ] Revamp the time picker
- [ ] Make moment.js a optional dependency by allowing to plug other time management libraries to give the functionality. Like date-fns (https://date-fns.org/).

These points are some objectives I have in mind, but they are not the roadmap for the version 4. This issue is just to let us discuss and define the roadmap together, so feel free to leave your comments with your thoughts!