Flame graph differentials

[jonhoo]

In brendangregg/FlameGraph@465ac0c, @brendangregg added support for differential flame graphs, which can be really neat for profiling performance regressions. They have a couple of effects on flamegraph.pl:

• They require tracking "delta" values in both TimedFrame and Frame when we generate them in flow.
• They mean that there may be an additional sample column in each stack line.
• They change the status line shown on hover for each frame.
• And they change the color palette.