Remaining todo items for supporting @Command-annotated methods

[remkop]

Some todos remain from PR #416:

• (WON'T FIX) Remove the @Command(addMethodSubcommands) attribute and add method commands automatically as subcommands if (and only if) the class has a @Command annotation. I changed my mind on this; it is useful to be able to switch this off if needed.
• (WON'T FIX) Make the CommandLine#addMethodSubcommands and getCommandMethods methods private to reduce the public API surface. I removed addMethodSubcommands (it wasn't used), but changed my mind on the getCommandMethods method and keeping it.
• Don't mix in options/positional params from the containing class @Command into the method commands.
• Command methods should be able to mix in the standard help options.
• Bugfix: unannotated method parameters should be assigned a positional index matching their parameter index (but skipping @Option method parameters)
• New feature: if compiled with the -parameters flag on Java 8 or higher, the method parameter name should be obtained with java.lang.reflect.Parameter#getName using reflection instead of the generic arg0, arg1, etc.
• Add documentation for @Command methods. Ideally add a recommendation on when to use which annotation style. (Location could be after 15.1 Annotated Methods or perhaps a separate page.)