list available sub commands
exclude sub-subcommands from subcommand list
add vim filetype hint to common-ptero
extract display_subcommands into common-ptero
whitespace
extract execute_subcommand into common
add delegate_to_subcommand to common

This is useful for subcommands that wish to delegate to their own
sub-subcommands.
Merge pull request #4 from mark-burnett/subcommand-refactor

Subcommand refactor