Native support for jsonnet input files

[ghost]

Similar to how DroneCI supports jsonnet: https://docs.drone.io/extend/config/jsonnet/

[mumoshu]

Yeah, I DO want this as I believe Jsonnet is much better than go templates for declaring complex setups.

Jsonnet code can be a little harder to debug/review/audit/etc so I'm thinking that it would play nicely with helmfile build -f helmfile.jsonnet that will produce a "rendered" helmfile.yaml from Jsonnet (#780 )

Happy to review any contribution related to this feature.

Implementation-wise, the only thing I'm unsure is that how to add custom functions to Jsonnet, like requiredEnv, exec, and helmfile values that is missing in the Jsonnet stdlib.