Add UnnecessaryStringOutput linter

This linter checks for unnecessary literal strings in script outputs.

For example, the following two pieces of code are equivalent, but the
later is more concise and hence preferred:

  %tag= "Hello #{world}"

  %tag Hello #{world}

Change-Id: Icde8c8ec176b042ae9b6ac0fd9daf9a2f732821b
Reviewed-on: http://gerrit.causes.com/42293
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>