[Feature Request] DatePicker: the 'format' attribute should accept function argument

### Existing Component
Yes

### Component Name
DatePicker

### Description
Currently the 'format' attribute (format of the displayed value in the input box) accepts a string argument containing [date formats](http://element.eleme.io/#/en-US/component/date-picker#date-formats):

Those formats only produce numeric date output (01/05/18), and does not allow for month names (01/May/18) or even timezone (01/May/18 PST). I'm assuming you chose this path to avoid i18n issues (although you do offer "AM/PM" output)

Perhaps if the format attribute also accepted a function argument (passing in the Date object) then I could return my own formatted string and not be limited to the date formats that you support.