Make it easier to plug in different parsers for file types.

[cjw296]

Currently you can either use the built-in ones or provide a callable for each parser call.
This is an issue if, say, you want to plug in a different yaml parser for all yaml usage, particularly when configurator is used within an application or library whose source code you don't want to modify.

This also came up on #7 ...

[cjw296]

@wimglenn ^^