Bring your own terraform binary

[armsnyder]

It looks like tfpluginfocs downloads the terraform binary each time it is run. We could work a lot quicker if we could tell tfplugindocs the location of a terraform binary to use instead.

I'd like to propose a new argument --terraform-binary [path] for this purpose. When specified, tfplugindocs would use that terraform binary instead of downloading it. (This is analogous to Helmfile's --helm-binary argument.)

[armsnyder]

#77 would solve my use case. Could a maintainer review that soon? (one line change) 🙏