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
Based on the two bits of code below, I think that behaviour is automatically handled if you use `help`.
https://github.com/scopt/scopt/blob/scopt3/src/main/scala/scopt/options.scala#L153
https://github.com/scopt/scopt/blob/scopt3/src/main/scala/scopt/options.scala#L208

Do you still want override def showUsageOnError = true?