Plugins: add map to store arbitrary data in Command

[rsteube]

Similar to Command.Annotations add a map[string]interface{} to store data that shares the lifetime of the Command.

related #2019

[maxlandon]

Allowing myself to add some content/arguments to this:

In the frame of the proposal/ideas put forth in #2019, and in order to make cobra completely agnostic to the "execution runtime" (that is, agnostic to how many times the commands might be ran), some side-projects such as the carapace one might need to store completion objects along with commands themselves, so that garbage collection of the commands is possible along with all their side-stuff.

This map would thus enable those projects to more easily and efficiently attach important stuff to commands.

The map would be actually just that: a map. Users would be in charge of initializing it and ensuring it is before access/use.
This would be strictly identical to command.Annotations.

To conclude, this feature request is very small and simple (adding a simple map struct field), but might have enormous use to many library consumers/side-projects.

Hopeful to see this included in the next release !