Parse version -json output and fall back to plaintext

[radeksimko]

Closes #13

I didn't add more tests because it seemed like we'd be just testing mostly stdlib internals - either os.Exec or encoding/json. I'm open to ideas on what else to test though.

Also I think that exposing VersionOutput from the Version() method would be a good idea, but I avoided that just to retain compatibility, for now. Also it might be helpful to leverage (not yet published) library for parsing the provider addresses too. Either way I can open a separate issue for this.

[kmoe]

This worked out better than I expected, as it looks like earlier Terraform versions don't error when -json is supplied to terraform version - they just ignore it.

Agree with testing approach here. We do need to add e2e tests for the providerVersions (see TODO in e2etest/version_test.go, but this can be done here or elsewhere. Let's create an issue if this isn't done now.

What would be the use case for exposing VersionOutput?

[radeksimko]

What would be the use case for exposing VersionOutput?

I don't have a strong use case for it actually, I just thought maybe in some contexts it would be useful to know the platform, but I guess if it already differs from the host platform then it's unlikely we'd even be able to execute terraform anyway, so I guess there isn't a strong reason 🤷🏻

[kmoe]

Great, let's keep a lookout for use cases until we're ready for 1.0, because now is the time to break things if we want to!

[radeksimko]

Created #147