This is a bit of a hack. The AbstractCommand class had to be pulled
apart to enable the ArgumentHandler to be imported by another plugin
without the other plugin having to extend AbstractCommand, which is
something it does not want to do because it is not defining a new
command, but just adding arguments.
There's too much passing of arguments in this implementation, but it
works. I'll see if I can simplify it.
Args no longer include the sub-command, just the args after the sub
command.
Improved tab complete so that it completes half-written commands
Fixed deprecation issue with visitor guard.
