Add some help from the command line

There does not seem to be any way to access command line help for the Error Prone compiler. The online documentation for the options are not very clear either.

The command line help should provide details of all the -XD and -Xep options as well as information about all the bug checkers, particularly their current severity after taking into account the BuiltInCheckerSuppliers settings, annotations for custom checkers, and any -Xep:... options.