Add additional flamegraph tests

[jasonrhansen]

Includes tests for the following from #63:

• Use a palette file
• Ignore bad lines
• Plot an empty trace
• Plot a file where frames get pruned
• Plot an inverted flame graph
• Include grey frames
• Test a non-sorted multi-reader input

[codecov-io]

Codecov Report

Merging #67 into master will increase coverage by 7.03%.
The diff coverage is 97.11%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   77.28%   84.32%   +7.03%     
==========================================
  Files          13       13              
  Lines        1268     1359      +91     
==========================================
+ Hits          980     1146     +166     
+ Misses        288      213      -75

Impacted Files	Coverage Δ
src/bin/flamegraph.rs	0% <ø> (ø)	⬆️
src/flamegraph/mod.rs	97.84% <100%> (+13.49%)	⬆️
tests/flamegraph.rs	96.05% <97.02%> (+0.81%)	⬆️
src/flamegraph/color/mod.rs	74.07% <0%> (+0.74%)	⬆️
src/flamegraph/merge.rs	94.38% <0%> (+2.24%)	⬆️
src/flamegraph/color/palette_map.rs	89.79% <0%> (+89.79%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 53ac6a5...6975001. Read the comment docs.

[jonhoo]

Looks good overall! Thanks for putting in the work :D
Left a few inline comments but they should be pretty minor!