opc: refactor _ContentTypesItem to instantiate
opc: refactor _ContentTypesItem to instantiate

Keep defaults and overrides as instance variables and change methods
from static to instance methods. It's still a one-shot object, but the
call structure is cleaner without having to pass defaults and overrides
around everywhere.

jean/python-docx,Nuos/python-docx,eruffaldi/python-docx,mohamedattahri/python-docx,ludoo/python-docx,SimplyAgree/python-docx,tanyunshi/python-docx,sk1tt1sh/python-docx,yurac/python-docx,python-openxml/python-docx,holli-holzer/python-docx,guilhermebr/python-docx