add `Date` extension

# Date

replace the string `datetime` etc to current datetime `2013-05-04 12:00:43` etc

Now it supports source strings as follows:

* `date` --> datestring eg: `2013-05-04`
* `datetime` --> datestring eg: `2013-05-04 16:03:05`
* `dt` --> same as `datetime`
* `time` --> current time `16:03:05`

You can also choose the datetime format:

* `Y-m-d` --> 2013-05-04
* `Y/m/d` --> 2013/05/04
* `Ymd` --> 20130504
* `D/M/Y` --> 4/5/2013