`borg --verbose prune` could print archive counts

[drws]

borg --verbose prune currently doesn't output anything useful during a normal run. It could print some archive counts:

• number of archives (in the repository)
• number of glob-matched archives (if --glob-archives is used)
• number of pruned archives (in a separate line at the end if unknown at the beginning)

[Konmous04]

Hi, I am interested in working on this issue. Is it acceptable if I try to implement this print feature?

[ThomasWaldmann]

@Konmous04 sure, thanks for helping!

Sounds like statistics, so guess there could be a --stats option?

Do a PR against 1.4-maint branch.