Usage as a month picker or minute picker only

react-datetime was designed to return the value as a Date object, that means it's better fit to handle complete dates with years, months, days, hours, minutes, seconds and milliseconds.

It's easy to not to use all the granularity and we can easily pick a year and a month by using the props, ignoring the smaller unit times.

However there are multiple requests about how to use the library only to pick a month (no matter the year) or as a minute picker.

I'm not sure if I would implement this feature, but in case many people keep asking for it, we would need to achieve kind of behavior:

* without breaking the current API, (maybe just adding a couple of new props)
* with minimal changes in the source code
* making all the possible picker available, not only months or minutes.

This ticket is about discussing and (maybe) implementing the feature