All plugins should by default have an auto-generated goal 'help'

All plugins should have a goal called "help" providing help on how to use the plugin. This should be auto-generated if it doesn't already exist as a goal in the mojo. The autogenerated help should typically list the goals (and settable properties) of the plugin, and probably also print out the url to the plugin's website (if available). This will most likely affect the plugin archetype as well, I guess.