Adding Non-US holidays in pandas.tseries.holiday
`pandas.tseries.holiday` has built-in only US holiday calendars. Is it possible to add other countries' holiday calendars to it? This US-centric module seems weird. 

Maybe add [holidays](https://pypi.python.org/pypi/holidays) module, so this is not hard-coded and maintained within pandas.

I could make a pull request for this if it sounds reasonable.