Allow the user to specify a local tarball.

This allows the user to create a godeb from a local tarball. This is useful
since 1.3beta1 is appearing on a different site.

Since I don't know if they're going to continue using that site, if beta2 will replace or sit
alongside beta1, if later betas will migrate back to the "main" site, etc, I'd hesitate to
write too much further glorious parsing code.

Apparently my auto-gofmt-on-save chewed on a couple of other bits of the code too.