* Make GITHUB_TOKEN optional
This also makes the token possible to pass in from CLI args.
* Add missing test fixture file
* Normalize exit codes and GitHub checking behavior
The exit code is now defined as 1 for unexpected or config errors,
and 2 for link check failures.
GitHub checking behavior has been tweaked to generate errors if
a GitHub-specific check cannot be performed because of a missing
token.
* Remove short flag for github token