Support scala implementation sjsonnet

[Qinusty]

I recently came across databricks sjsonnet as an alternative interpreter for jsonnet, it is quite fast (faster than go in my use case) but is missing some features (std.slice is one example, but that is missing from the jsonnet docs).

This can be added as a non-default tool option and I aim to improve the compatibility between sjsonnet and standard jsonnet files using the full functionality of the go-jsonnet tool. I have raised a tracking issue for these incompatibilities over on the sjsonnet project.