Create a dice rolling plugin akin to /marqee

There should be a plugin that abuses XEP-0308 to simulate a roll of dice. Bonus points for using U+2680..U+2685 (Unicode DIE FACE) for display, and for allowing multiple dice to be rolled at the same time, i.e.: /roll 3

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• Changesets:
  • Revision d55ce5b9 by mathieui on 2016-07-04T23:15:04Z:

Fix #3197 (add a dice plugin)

also add some doc for the marquee plugin