Add n_clicks_previous to buttons to mimic new dash-html-components logic

[mjclawar]

Discussion

As discussed in plotly/dash-html-components#37, buttons are much more convenient when the user can determine which button was clicked in the case where multiple buttons are used for a callback.

Adding an n_clicks_previous prop to at least RaisedButton and FlatButton would clean up a lot of logic in downstream code.

[coralvanda]

Although there are two updates to props, the single click event fired off appears to hit the server only one time per button press. Watch the POST updates in the console in this gif as each button is clicked.