Feature request: Add MR priority support

Created by: t0ny-peng

Suppose I have 5 MRs assigned to marge-bot, it'll by default pick the oldest one. However, if one is urgent and should be merged before others, currently there's no way to specify the sequence.

I would suggest such change:
After getting all MR lists, check if any of them has label merge_first. If so, merge it first. Otherwise, merge the oldest one.