feat(datepicker): add datepicker-mode

[Update with: https://github.com/tomchentw/bootstrap/commit/00db638b35f43ef73d74f5fa1131c3a4737c06c9]

Thanks for creating SUCH a great project!!

add `datepickerConfig.datepickerMode` and attribute `datepicker-mode` on `datepicker` directive:

* accepted values are one of names: ('day', 'month', 'year')
* `datepicker-mode` is two-way bound attribute
* add `datepicker-mode` to README