Try to make compatible code with direct Python call ; reduce the hard link of the code with the cli

[gallofeliz]

Hi,

I started to use this code to backup my repositories, but I use it directly with Python.

The main problems :

• Forcing to pass an USER in the command (after I make a args.user = os.environ['USER'] to change it, it's okay)
• In case of "error", the script will exit ; what I don't want
• Logs are directly writen in stdout and stderr, so I can't format them

I tryed in this MR to make the smaller changes as possible with the same behavior for CLI :

• args can be passed to parse_args()
• Use of logging module to be able to change formatting
• Errors are now Exceptions and catched by the cli file to be transformated to a log + exit

We need to test all is okay, but I can detect a small change : log time uses now 3 decimals for microseconds and no more 6.

Regards,