Add support for extensible commands

[DannyBen]

This PR lets us specify that we want the generated bash script to look for another executable when an unknown command is used. This is similar to how git works.

It allows two modes:

• Extension mode: Similar to git, where any calls to mycli anything, will look for an executable named mycli-anything in the path and call it if found.
• Delegate mode: Where any unknown command will be delegated to a predetermined executable.