Add support for add-ons to define commands using the new argument
`-c` or `--command`.
Commands behave as follows:
- Passing in a command enables background mode without the need to pass
in `--background`.
- All arguments following the command are passed to the command
(without the need to use the `--` argument).
- Add-ons can define their own commands via
`bpy.utils.register_cli_command` (see examples in API docs).
- Passing in `--command help` lists all available commands.
Ref !119115
Campbell Barton
Bastien Montagne