Allow creating POJO config classes anywhere without requiring the plugin user to specify an implementation element

This is really important for several reasons:

• I don't like to clutter my main plugin package with POJO classes. For the Cargo m2 plugin I have 7-8 POJO classes and I'd like to move them to a different package
• I need to reuse existing POJOs from another jar and it's just too stupid to have to duplicate all the code or write wrapper classes with only getter/setters.

I think the best solution would be to accept @implementation javadoc tags that would map a parameter to an implementation. This would need to work not only for Mojo classes but also for POJO classes being used for configuration.