Implement pipeline upload --replace

[GMNGeoffrey]

When running buildkite-agent pipeline upload --replace right now, you just get panic: Replace not implemented. Taking a look at the code this looks pretty trivial to implement. It would be exactly like prepend except without including the existing steps. Is there something I'm missing here? I guess that wouldn't account for the caveat that replace will not replace already-running steps, but AFAICT the steps are being run sequentially, so a later step should never be running anyway.