rework return values from _parse_* function to return true if the value was processed false otherwise, this simplified the logic and got rid of the pulling and clearing of the missing fields from option groups.
add TriggerOff and TriggerOn helper functions and some tests for them
add shapes example of multiple callbacks in order.
allow specification of callbacks that get executed immediately on completion of parsing of subcommand
add tests for enabled/disabled by default
add _get_fallthrough_parent. To get the most appropriate parent to fallthrough to
add enabled and disabled by default functions
add positional_arity example
Add a pre_parse_callback_ for apps. The Pre parse callback takes an argument for the number of remaining arguments left to process, and will execute prior to parsing for subcommands, and after the first option parse for option_groups.
