#47 Rework subcommand handlers like the exit handlers.

[Rick McGuire]

The subcommand handlers could use a rework along the lines of what was done with the exit handlers. This includes

1) Keeping a cache of resolved subcommand handlers for
quicker calling.
2) Allow subcommand handlers to be established on an interpreter instance basis as options. It should be possible to specify multiple handlers for a single interpreter instance.
3) An infrastructure for command handlers similar to the exit handlers should be created for easier management.
4) The "hard coded" system handlers should be reworked into the handler infrastructure so that they are no longer special cases. These handlers should be added to the case by the platform-specific code during instance options processing.
5) Potential -- allow an instance to disable the system command environments, if desired.