[Feature request] add a event when the date clicked ( range mode )

[liurongwei]

What problem does this feature solve?
use range mode to select date range , when want dynamic controller the select range depend on the first date picked , there is no way to make the second date selectable in 30 days.

What does the proposed API look like?
add a event fired when the first date been checked .

[mengxiong10]

#385

[mengxiong10]

same as #429 . I'll add it in next version.