Implemented "Today" button

[bojant987]

Added a "Today" button at the bottom of datetime picker.

Description

Just added a showTodayButton prop ( default false ) for conditionally rendering "Today" button, along with styles for it. Also included it in date validation, so it can be disabled if today is not pickable.

Motivation and Context

I initially implemented this because we were needing this feature for our project at GogoAir. Thought it might be nice to share it with the rest of the folks, it's a common feature for date pickers.

Checklist

[x] I have not included any built dist files (us maintainers do that prior to a new release)
[x] I have added tests covering my changes
[x] All new and existing tests pass
[x] My changes required the documentation to be updated
  [x] I have updated the documentation accordingly
  [x] I have updated the TypeScript 1.8 type definitions accordingly
  [x] I have updated the TypeScript 2.0+ type definitions accordingly

[artemu78]

that would be great feature, i wish it will be included one day