Add 'prune' command

[fd0]

In #459, the optimize command was removed due to a bug. This issue is about adding a new command called prune which basically does the same as optimize, just with a better name (and hopefully without the bug).

[squarism]

Is this similar to deleting snapshots? I'd like to subscribe/watch an issue that represents when deletes are implemented if I understand restic correctly (which may not be the case). As of right now, are repositories append only?

[fd0]

This issue is about adding the prune command, which is one part of regularly removing old snapshots. I'd suggest subscribing to #51 or #18.

And you're right: At the moment, repositories are append-only. We had a command similar to prune for a while, but it turned out that there was a grave bug in the code so I removed it for now. Better safe than sorry.

[fd0]

I've nearly finished the implementation of a prune command and need some feedback on how to specify which snapshots to remove/keep, please add your comments to #572. Thanks!