Improve usage of Rhino: reuse scope, enable optimization

Instead of creating a new scope for every check, we can reuse the scope
once it is initialized.

Set Rhino optimization level to maximum for JSHint evaluation in order
to create high-performance JSHint bytecode.
Use JsonObject for configuration

Instead of a dedicated class Configuration, provide the JSHint config
directly as a JsonObject. That will also allow to accept config files.
Adjust UI to changes in JSHint (JsonObject for config)