ENH: Add 'add_metaclass' class decorator.

Taken from a new addition to the six library (again, license is in the
LICENSE directory).
CLN: Make ExcelWriter more pluggable

Make ExcelWriter an ABC and add ExcelWriter config to core/config_init

ENH: Allow Panel.to_excel to pass keyword arguments
Merge pull request #4750 from jtratner/refactor-excel-writer

CLN: Make ExcelWriter more pluggable