Ensime support

I was wondering if there is any simple way of adding support for ensime? As reference, the following link references support for other build tools for it: [http://ensime.github.io/build_tools/](Ensime build tools).

Start with the Ensime build-tools reference linked in the issue and review how rules_scala currently integrates with Bazel. Identify the files and entry points involved before defining the supported Ensime workflow. Done means a documented, tested Ensime integration, but the issue does not specify the expected behavior or test locations.

Written by the indexing model from the issue text.