improvements to complex mojo configuration

currently, you can specify an object in the configuration that allows nesting multiple levels of config. This is helpful, but it would be good to be able to validate those portions of the configuration as well as set default values, and make them extendable by making them fully fledged configured component requirements.

basically:

• be able to specify expressions and default values within the fields of the object, not just the top mojo, as long as it is in the same source tree as the mojo (like extends)
• to be able to put a polymorphic object in there without an implementation given in the pom. This might require selectors, and so might be best left until a later version (se elinked issue for a use case)

I haven't tried, but it might already be possible to do this (from components.xml), and we just need to wire up the tools to handle it.