Default option group: make it globally configurable

[janluke]

This is a minor enhancement, a refinement, to implement (maybe) in the future.

Specs

• The name and the help of the default OptionGroup should be globally configurable.
• Important: as it is implemented now, if the command has no option groups defined, the default group name should be just "Options" and the help should be empty.

Notes

• There should be an analogous issue relative to subcommand sections; these configurations should be released together. Subcommand sections don't strictly need this, because no command is added automatically by Cloup (as it happens in the case of options with --help), so if the user wants to change "Other commands" to something else, (s)he can define another a section and leave the default one empty.

[janluke]

Related: #15. This feature can be implemented with a context attribute

default_option_group: OptionGroup = OptionGroup('Other options')

[janluke]

I don't feel this deserves yet another setting. If one really wants to change the default, one can already override the get_default_option_group method.