Multilevel command execution support

Basic support for running multilevel subcommands. Completion, help, and
others still not working.
Add jeffreyroberts' example sub sub directory
Add support for multi level subs to sub-command
Add support for multi level subs to sub-help
Make help work when called implicitly

If use runs a sub command that is a directory, show its help.
Use non unicode character to indicate directory

Since previous unicode arrow used to have problems with alignment.
Add completion support for multi level subs

Zsh completion still pending.
Add doc file to top level sub directory
Modified prepare script for sub sub commands
Add more contributors to Copyright notice
Remove subdirectory 'sub' from share directory

I never understood the point of it.
Remove unnecessary contents from doc.txt files
Document multi level subs in README file
Fall back to Bash's default completion

If sub does not provide any completion for the current command, use
Bash's default, i.e., file names.
Suppoert multi level sub 'sh' sub commands
Add a python implementation of sub script

Initial step of migration to python.
Just rename subpy to sub
Implement sub-commands as part of 'sub'

Remove the sub-commands standalone script and implement its logic inside
the main script.
Fix sub-init to support containers
Change sub-help to use the internal "commands"