Add dayClicked event

[ravarage]

add dayClciked event, add event its important, to trigger when a user does a certain action when user clicks on a day it gives store back to function event and it can be accessed using event.detail or you can access date in store as before up to you

[divStar]

This looks awesome, but I think 1. the docs should be updated (/docs folder) and perhaps the event should be just picked or something in order to integrate it into MonthPicker and YearPicker, too.