[FEATURE] Add comments to YAML config printed with `-printcfg`

[Aurélien Rainone]

Adding comments explaining what the settings are for, and possibly some choices (where applicable) would be really helpful for users.

Comments will need to be added in embedded gitmux.yml file.
We can remove the tmux.DefaultCfg variable, it will be unmarshaled from the embedded file (when -printcfg has been given)