"Remotely" show/monitor status of running backup

### Why?
Especially when running large backups one might want to query the status of a backup while it is running.

One should be able to get the status, remaining time of a running backup without interrupting it.

#### How?

I've titled this with "remotely", because usually when your backup runs in a cron job e.g. you don't can/want to use `-p` as you usually redirect its stdout/sterr output into a log file.
That's why I'd propose to integrate a way so that one can run a borg command (`borg check status`??) to get the status of (all) other running backups on the machine. Alternatively also the repo could be given, so that this is used to get the status of the running backups (maybe by still connecting to the running process locally)…

So the usage should be:
1. Backup is running.
2. User starts a new terminal, enters borg command and gets the status of the backup… (either continue – like `tail -f` – to update the status or just exit and print the result once – like usual `tail`)

### What?

In a simple way it could look like [pv](https://askubuntu.com/questions/215505/how-do-you-monitor-the-progress-of-dd#answer-215590) and show:
* what is running (archive name)
* progress bar and/or percentage
* "speed"/data rate (byte/sec)
* amount of time spent
* data processed
* (maybe) data to process or whole backup size
* estimated time until end (probably the most important stat)
* maybe some borg-specifc things like chunks
* currently processed file (like `-p`)