Detect compare URL automatically based on .git information

[rcmachado]

When running the changelog command we can detect and build the compare URL based on previous versions already specified in the markdown file.

But if we have a brand new .md with only an Unreleased section, the detection becomes unreliable and we require the user to pass --compare-url flag. If we are operating in a git checkout, we can automatically detect it based on the remote configuration.

The goal is to implement this for GitHub repositories in the first place.