GitLab.com as a host was always assumed, but can now explicitly be set
to a host of the users chosing based on a flag. The transport protocol
needs to be added as testing if HTTPS is supported is not something
Snippet should to in my opinion.

The trailing slash is added, as that's required by a downstream
dependency.

While creating this commit, the vendorred go-gitlab is updated to keep
up with th changes.

Closes https://gitlab.com/zj/snippet/issues/1