Make control over response parsing more fine-grained

[adamretter]

This gives the user much more control over the response parsing, including the ability to choose the parser for the response when they know better than the response's content type.

Probably needs some editorial to make the language more human friendly, but assuming people are happy with the approach I think the editorial can come in a later PR.

Closes https://github.com/expath/expath-cg/issues/108