Solidify the conflict warnings on startup.

* Create simple configuration for how/when to warn.
* Add the ability to disable the conflcit.
* Display where ivy reports are, and how to disable the
  warning when issuing it
* Only display conflcit warnings (by default) for
  modules which are *directly* specified in a build.  We
  don't care as much about transient dependencies.
  Also, we don't care if sbt plugins were built with older
  sbts, but we do care if a plugin somehow bumps the desired
  sbt version for a project.
* Only display conflcit warnings for user-facing ivy configurations
  (Test, Compile) rather than all of them (scala-tool, etc.)