Allow comments in .terraform-version file

[OJFord]

Using syntax:

# this whole line is a comment
1.2.3
# this is also a comment

but not inline comments.

Closes #283

[jgrumboe]

Hi @OJFord I would vote against comments as there are probably also other tools relying on this file in the existing format. And there’s also an open issue at the setup-terraform action which could also rely on the file. https://github.com/hashicorp/setup-terraform/issues/208

Additionally, I checked .ruby-version and .node-version and both also don't support comments as that supports the simple "cat" use case like rbenv use $(cat .ruby-version) for example.

[mputilin]

Useful feature. Please merge.