Partial fix for #281- covers the `*@users.noreply.github.com` case.

The `users.noreply.github.com` domain comes from users
making use GitHub's 'email privacy' feature.
Since the noreply address doesn't accept email, a user can't link
their preferred Gravatar to this type of email address.

The upside is that this address is easily parsable into:
* (optional) Github User ID
* Github User Name

We can use either to create the associated avatar link
without actually having to do an additonal request
against Github's user search API.