Adds support for the terraform 0.12upgrade command

[gdavison]

Adds support for the terraform 0.12upgrade command

[radeksimko]

👋 Hey Graham,
What is the intended use case for this command?

In general terraform-exec is used for automation (i.e. usually unattended execution) and from what I can tell the upgrade commands are not intended to be used in a "fire & forget" way, because there are no guarantees that the upgraded config will be actually usable and it may require further manual changes.

[gdavison]

I'm using it for an enhancement to https://github.com/katbyte/terrafmt. It would allow users to apply terraform 0.12 to any terraform blocks in Markdown or Go files.

[kmoe]

Thanks for this. The use case is valid and the implementation sound. Thanks for including a unit test.

Happy to merge when an end-to-end test is added. There's a section in CONTRIBUTING.md describing how to do this. If you don't have time, please let me know and I'll be happy to add it.

[kmoe]

LGTM