Add `<DateTimePicker>` interactive/input component

A new <DateTimePicker> interactive component can use in <Actions> and <Input> layout block. It also can use as an input component by setting label prop.

Close #276.

<Modal title="Snooze">
  <DateTimePicker label="When should we remind that?" value={Date.now() + 300000} required />
  <Input type="submit" value="Set" />
</Modal>

[x] Add <DateTimePicker> interactive component
[x] Add tests
[x] Update demo
[x] Add docs

yhatt / jsx-slack

Add `<DateTimePicker>` interactive/input component #278