Pass 'breaks' to geom_histogram

[jgabry]

This PR just adds a breaks argument to histograms that can be used as an alternative to binwidth. Turns out to be useful sometimes.

[codecov-io]

Codecov Report

Merging #148 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   98.78%   98.79%   +<.01%     
==========================================
  Files          30       30              
  Lines        3879     3891      +12     
==========================================
+ Hits         3832     3844      +12     
  Misses         47       47

Impacted Files	Coverage Δ
R/mcmc-distributions.R	100% <ø> (ø)	⬆️
R/mcmc-diagnostics-nuts.R	100% <ø> (ø)	⬆️
R/ppc-test-statistics.R	99.41% <100%> (ø)	⬆️
R/ppc-errors.R	99.51% <100%> (ø)	⬆️
R/mcmc-diagnostics.R	97.63% <100%> (+0.02%)	⬆️
R/mcmc-recover.R	100% <100%> (ø)	⬆️
R/ppc-distributions.R	100% <100%> (ø)	⬆️

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[jgabry]

@tjmahr I requested your review on this to keep with best practice, but I know you're short on time these days and this is a super simple PR, so if you want me to just merge it that's fine. Otherwise review it whenever you have the chance, but no rush.

[tjmahr]

Yup, super simple PR.