Allow github organizations not named maptime.

This adds an additional option in `_config.yml` to specify the github organization to use when creating URLs to the repo. By default, it's 'maptime' which should just work most of the time, but if the organization is something else, a few broken links would be generated.

It would be nice if there a way to define a variable in Jekyll like 'site.repo_url' that uses the variables in config and can be referenced in the templates, but I couldn't find an easy way to do that.