Allow for using a different masthead/opening paragraph

[brettcannon]

Basically what I'm after is the ability to use my own opening bit after # Changelog instead of the standard one that Keep a Changelog provides.

[Colin-b]

Hello @brettcannon , what function are you using exactly?

[brettcannon]

Nothing yet, but I was contemplating using the to_dict()/from_dict() and the docs don't seem to suggest that a custom opener would roundtrip.

[Colin-b]

Indeed that would make sense. Now I am wondering if this info should be returned by to_dict or be an additional parameter.