Command Not Found Hook

Ion should do as other shells and offer a means of executing a custom handler in the event of a command not being found. Common use case is for Linux distributions to do a search of their software repository for a package that contains that command.

As for how we do it, I'd think that having a variable define the command to execute would be a good idea, and failing to find that, a function. Something like let COMMAND_NOT_FOUND = pkgfile, and if a command isn't found, simply execute pkgfile <command> in it's place.