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
Can you add a line:
override def showUsageOnError = true

to keep the current behaviour of showing help if no input files are given.