Added duplication vector for burr_plate() module.

[jfjlaros]

Perhaps this functionality is already implemented in some form or another, if so, please decline this pull request.

I have encountered a number of puzzles that contain multiple copies of the same piece. Instead of copying the piece specification, it would be nice to have a way to indicate which piece should be duplicated and how many times this should be done. In the example below, we duplicate piece 2 once and piece 3 four times.

burr_plate([
  ["xxx|.x.", "...|.x."],
  [".xx|xx.", "...|.x."],
  [".x.|xxx", "...|x.."],
  [".x.|xxx"],
  ["x..|xxx"],
  ["x.|xx", "..|.x"]], dup=[[1, 1], [2, 4]]);

I hope you find this useful.