Add the list_args function

[galactics]

I needed a way to produce a list of all the arguments available in order to do autocompletion with the commands I develop.

The proposed implementation doesn't break the structure of the file, but at the price of a little duplication. This is not ideal, but I wanted to show the principle quickly and efficiently.
I'll be glad to iterate over a better implementation, with better integration.

[galactics]

Hello to anyone listening,

I don't have any answer for the two years since this pull request has been posted.
Is there anything I can do to make this interesting to anyone ?

Otherwise I will just drop it