adding more tests to make refactoring the way custom commands do args easier

some of this probably was overkill; I should've done it the way I did it in custom_usage.t
but perhaps this will be useful as well
refactored custom command args into a class-let

this should allow me to easily add help text on a per-arg basis
and hopefully, eventually, validation too
can now specify descriptions of individual args for custom commands

this necessitated creating a new role, App::VC::BiColumnar, which handles the formatting
-	used now by both CustomCommand::description and Command::commands::execute
also had to tweak my test class a bit:
-	to properly report die errors
-	and to properly count blank lines before help options