choosenim remove needs "--all" or doc equiv

[gwiede]

Currently, choosenim won't remove the "current" nim. For someone wishing to completely uninstall everything choosenim downloaded/placed onto their computer, there really is no good option, nor do docs appear to cover that scenario.

If the intended uninstall is just for the user to rm the "NIMBLE_DIR", just state that clearly in the docs (and explicitly incl. what cmd would be needed for then-default dir).

Alternately, as "choosenim update self" is a thing, why not "choosenim remove self" (then have the user confirm they're okay with scorching the earth under themselves, and only then rm everything).

As always, better to provide a cmd even if it simply chains to an rm script for now, because eventually it will need to do more (remove auto-added path additions, for example), and that's where the "doc the rm cmd needed" historically falls down and goes boom, usually inflicting pain on both users and project volunteers. Just saying.

Not a demand just a feature request (and a sidenote there's a problem brewing with trying to remove "#devel" which means this code will need to be touched in near future anyway...).