add extractor methods for CmdStanMCMC objects (from CmdStanR)

[jgabry]

Adds extractor functions (nuts_params, neff_ratio, etc.) for models fit using CmdStanR. Also added tests, but they are skipped on CRAN and if CmdStanR is not installed.

[codecov-commenter]

Codecov Report

Merging #227 into master will decrease coverage by 0.70%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
- Coverage   99.28%   98.58%   -0.71%     
==========================================
  Files          31       31              
  Lines        3927     3958      +31     
==========================================
+ Hits         3899     3902       +3     
- Misses         28       56      +28     

Impacted Files	Coverage Δ
R/bayesplot-extractors.R	69.47% <12.50%> (-28.99%)	⬇️
R/mcmc-diagnostics-nuts.R	100.00% <100.00%> (ø)
R/mcmc-intervals.R	99.20% <0.00%> (+<0.01%)	⬆️

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[tjmahr]

Works on my machine. But do update NEWS.md

[jgabry]

Thanks, yeah I forgot to update NEWS. Just added a news entry.