FR: add --threads option

[thibaultmol]

By default my Vykar runs at 4 threads but sometimes I'm like "you know what, I'm not at my pc for a while, go ham" and would like to then do
vykar backup --threads 16 which then overrides the amount of threads set in the settings file

[m3nu]

You can already change this in the config file. But I thought about having the most common options that change from run to run as CLI arg. Like --compression.