Add the configuration options used by the plugin

[marmistrz]

Before that change, VS Code would complain about unknown configuration options if those options were added to settings.json.

This is properly recognized by VS Code. The CI failure seems unrelated.

[dragos]

This was supposedly done already in #71 but I have no idea what happened to that commit. I can't find it in git history, even though the PR was merged. :-/

[marmistrz]

For some reason today the ensime language server refused to start when using this package.json with the current release, so remerging your change may be a better idea.