Add a few convenience methods

[johnynek]

No description provided.

[codecov-io]

Codecov Report

Merging #17 into master will increase coverage by 0.44%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   90.29%   90.74%   +0.44%     
==========================================
  Files          12       12              
  Lines         206      216      +10     
  Branches       13       13              
==========================================
+ Hits          186      196      +10     
  Misses         20       20

Impacted Files	Coverage Δ
core/src/main/scala/com/stripe/dagon/Dag.scala	94.35% <100%> (+0.49%)	⬆️

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[erik-stripe]

looks good, 100% coverage, shipit. 👍