add mojo to projecthelp to describe supported plugin parameter expressions

People wanting to develop mojos need some help determining what plugin parameter expressions Maven will support. The projecthelp plugin would be a good place for this type of mojo, which would simply read the plugin parameter documentation used to diagnose plugin configuration errors (paramdoc files), and do some simple pattern matching on the expressions before outputting information about expressions and how to affect their values, not to mention descriptions of what they do.

This will depend on more complete paramdoc information.