Command line option for more verbose debug logging?

This is a bigger issue for bazel in general, but it would be nice to be able to optionally have verbose logging in custom rules.

TwitterScroogeGenerator would be a good place to have some, and the spirit of this comment https://github.com/bazelbuild/rules_scala/blob/master/scala/scala.bzl#L102

Start with the TwitterScroogeGenerator entry point and the referenced section of scala/scala.bzl around line 102. Clarify how an optional Bazel command-line setting should reach custom rules and what verbose output is required; done means the generator can emit that logging without changing default behavior.

Written by the indexing model from the issue text.