add clobber, skip-existing, update parameter to glab release download

Problem to solve

I want options to decide what to do if the file i'm trying to download already exists:

1. clobber (replace)
2. skip-existing (nothing/skip)
3. update (replace if newer)

Proposal

Add parameters to the command. gh already has the first two.

Further details

I want to be able to update release, and know what happen in cases of filename conflict.

Links / references