Allow github-request authentication.

Prior to this commit, github export requests would frequently hit the
rate-limit established by github's API.  Other than slowing down
bugging-out's requests to a crawl, there's not much that I could do
about this.

However, Github's API establishes a *much* higher rate-limit quota for
authenticated requests than it does for unauthenticated requests.  This
suggests an easy workaround for the problem mentioned above: allow users
making github requests to specify their username/password to get the
(much) higher rate limit.

This commit does this.  --username and --password options specified on
the command line are now passed to GithubIssueRequester for use in
authentication.

Issue #16