Respect terraform -chdir option

[iTaybb]

It would seem that tfenv does not respect terraform -chdir global option, e.g., won't look and use the .terraform-version on the changed directory.

Solution should be straight-forward, if -chdir is configured on the commandline, look there instead of the working dir for a.terraform-version file.

I'll be happy to create a merge request for that.

[OJFord]

Oops, sorry I didn't see this before I created duplicate #301. I've also submitted a PR #303 - would be happy to have your review/feedback of that @iTaybb.