Add un-required arguments: username/password.

Github's API currently allows users to drastically increase the rate-
limit they're subject to by authenticating as a Github user.  Currently
I don't make use of this nice feature.  However, I would like to soon.

This commit takes the first step towards supporting this authentication
by providing arguments where a user can specify their Github username
and password.  Currently, the provided values aren't used for anything.
This commit just puts the plumbing in place to support this in a future
commit.

Issue #16