Add A/B comparison support

Hi! It would be nice to have an A/B mode for speedscope.

Let me clarify what I mean by this. Suppose I have a program, and gather its initial profiling data. This would be the "A" output. Then I go and make a bunch of changes, and gather some new profiling data. This would be the "B" output. Then I'd want to compare what changed between A and B.

This is a very common use case, and I wonder what your thoughts are about adding some native functionality for this.

Thanks!