Upgrade scopt from 2.1.0 to 3.2.0

This pull request builds upon this one: https://github.com/twitter/scrooge/pull/114

 - Adding unbounded() calls to parameters that can take multiple options.
 - Also migrates scrooge-linter Main to scopt 3.2
You can test this with 
./sbt scrooge-generator/assembly
java -jar scrooge-generator/target/scala-2.10/scrooge-generator-assembly-3.17.0.jar 

I was looking at scopt docs at https://github.com/scopt/scopt