Customize PercentLiteralDelimiters cop

Square brackets emphasize that the %{i,w,W} calls return arrays.

Change-Id: Idf2f271e08f2c0ae72e9034d9762bcba1571d8f9
Reviewed-on: http://gerrit.causes.com/36705
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>