Better handling of multiple profiles

[bjorn3]

Only crox can currently handle multiple profiles at once. It would be nice to be able to aggregate multiple profiles into a single summary or flamegraph or to be able to take the difference between the aggregated profiles of all rustc invocations for two separate cargo invocations.

[bjorn3]

Also nice would be to allow summarize diff to accept two directories and then do pair-wise comparisons based on the crate name.