Add convenience method to Plan and return JSON as single func

[paultyng]

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.

[paultyng]

The detailed exit code was handled in #55, but leaving this open as an atomic way to Plan and get plan JSON would be nice, possibly as a convenience method.

[paultyng]

This is somewhat related to #10, but specific to plan JSON, not state.