Add convenience method to Plan and return JSON as single func

One option is supporting the exit code:
https://www.terraform.io/docs/commands/plan.html#detailed-exitcode

The `Plan` signature could be changed to: `Plan(...) (changesPresent bool, err error)` or something similar to surface the exit code data.

Another option is just returning the actual JSON plan always from the `Plan` method. In programmatic usage you probably will want it frequently anyway.

We could optionally do both of these in the return values.