[feature] Differential flamegraph

### What version of pprof are you using?

master (236ed259b199815a270edb9f8702bd0102e2e8fa)

### What operating system and processor architecture are you using?

OS X 10.14, x86_64

### What did you do?

I was hoping to visualize the differences between two profiles as a [differential flamegraph](http://www.brendangregg.com/blog/2014-11-09/differential-flame-graphs.html) with something like:

`go run github.com/google/pprof -http localhost:8080 -diff_base A.pb.gz B.pb.gz`

### What did you expect to see?

A flamegraph with colors that indicate relative changes between the two profiles.

### What did you see instead?

Nothing, the flamegraph view errors out with `TypeError: null is not an object (evaluating 'd.c') in flamegraph:6021`.

The other view types (e.g. Top, Graph, Peek) seem to work and display percent differences/colors. I haven't looked into how difficult it would be but am interested if you would consider a patch to this effect.