Raid: ability to pass e.g. --chunk to mdadm

[fillest]

As with RAID_LAYOUT, it's desirable to be able to specify --chunk too.
I even think it would be good to have an arbitrary "extra args" field that can be passed to mdadm as is - to proactively cover other possible (and future) needs with any other extra flags.

[cyberplant]

I think this can be done easily after my PR: #85