Better support for JSON files describing commands metadata

[zuiderkwast]

• Improved JSON 'arguments' fallback in gencommand.py
• Update generated command table from latest Redis
• Support the output of redis utils/generate-commands-json.py
• Update comments in gencommands.py about the JSON file format

[zuiderkwast]

Good comments about find_keys differences.

THanks. What abour "keynum"? It's the same difference. I didn't want to repeat the code logic in the comments though.

[bjosv]

THanks. What abour "keynum"? It's the same difference. I didn't want to repeat the code logic in the comments though.

I think its fine with the added comments here.