Added midje dev dependency.
Complete rewritting plugin to using cucumber.api.cli.Main.

This resolves a lot of issues on relying on cucumber-jvm's
internal API which plague the previous version of lein-cucumber.
With this rewrite, the focus for this plugin can then be on
making the plugin more awesome, rather than making it
compatible with the latest cucumber-jvm version.
Bumped up major version to mark this complete rewrite.
Updated usage section in README.md.