Add support for terragrunt show

[dgethings]

Not sure if it's a bug or not but

terragrunt-github-actions/src/main.sh

Lines 205 to 208 in 259fc9e

	*)
	echo "Error: Must provide a valid value for terragrunt_subcommand"
	exit 1
	;;

means subcommands only supported by terraform (i.e. show) are not passed "down" to terraform.

I'm not sure about the best way to fix this. This PR adds support specifically for the terraform show (sub)command.

Perhaps a more general fix would be to just handle whatever terraform returns. But I really don't know if that's a good idea or not.

Anyway, here's a PR adding some functionality. HTH.

[dgethings]

Did anyone get some time to review this PR? I've been using my fork since I wrote it and it has worked perfectly for me every time.