Datetime check function prop

[phyzical]

Hey @mariomka

Sorry for the wait had been busy. Update from previous pr https://github.com/mariomka/vue-datetime/pull/226

• added new prop datetime-disabled-checker, this provides (year,month,day,hour,minute,second) as params to the provided function
• there is an example of how ranges could be implemented in the demo changes
• added functionality to disable am/pm when no selections available
• added tests

[ciryon]

Great feature, would be perfect to have it merged!