add ShowState, ShowPlan

[kmoe]

terraform show -json PATH will output the state file or plan file at PATH in the corresponding tfjson format.

Since we must know ahead of time whether we're returning *tfjson.State or *tfjson.Plan, we now require the user to call either ShowState() or ShowPlan() if they are supplying a PATH.

Add end-to-end tests. Negative compatibility cases will be covered in #58.

[kmoe]

@paultyng any idea why the test fails on Windows?

[paultyng]

I wonder if we should just write up a runTerraformCmdJSON as we have a lot of these stdout unmarshalling patterns.

[kmoe]

runTerraformCmdJSON() could work, but we're marshalling into different JSON types each time...

[paultyng]

yeah, you would pass the pointer just like json.Unmarshal