add git-like subcommand deferring mechanism

[TeddyDD]

This commit implements a simple mechanism of deferring unknown
subcommands to other executables in $PATH.

When users executes kks foo kks will try to run kks-foo command.

Example:

$ cat ~/.local/bin/kks-date                                                                                                                                                                                  
#!/bin/sh
date $@
$ kks date -I
2021-11-12
$ kks grep
...

Just an idea, feel free to ignore if you don't like it (It can be implemented as wrapper script by user) :)

[kkga]

I quite like the idea. Thanks!

Let me test this a bit before merging.

[kkga]

Looks and works great. Found a small typo in error naming...

[TeddyDD]

Whoops, fixed.

[kkga]

Great, thanks again!