Add support for differential flame graphs

[jasonrhansen]

Implements #25

[jonhoo]

Looks like clippy is complaining.

[jasonrhansen]

The first clippy fix is simple, but it's complaining about the complexity of from_sorted_lines. How do you want to break that up?

[jonhoo]

Re from_sorted_lines, ultimately I want the code that draws each frame to be in its own function, but #52 already makes some headroom we can build on there. For the time being, just add

#[allow(clippy::cyclomatic_complexity)]

[jonhoo]

Also, awesome work! ❤️

[codecov-io]

Codecov Report

Merging #60 into master will increase coverage by 1.6%.
The diff coverage is 85.34%.

@@            Coverage Diff            @@
##           master      #60     +/-   ##
=========================================
+ Coverage   75.78%   77.38%   +1.6%     
=========================================
  Files          14       15      +1     
  Lines        1210     1287     +77     
=========================================
+ Hits          917      996     +79     
+ Misses        293      291      -2

Impacted Files	Coverage Δ
src/bin/flamegraph.rs	0% <0%> (ø)	⬆️
src/flamegraph/color/mod.rs	73.33% <100%> (+1.67%)	⬆️
src/flamegraph/merge.rs	91.66% <100%> (+6.81%)	⬆️
src/flamegraph/mod.rs	84.34% <100%> (+1.73%)	⬆️
tests/flamegraph-differential.rs	90% <90%> (ø)

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