Add iter1 option to mcmc_trace() to start iteration counting after warmup

[mcol]

Fixes #14.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@8a016cb). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #155   +/-   ##
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  Coverage          ?   98.74%           
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  Files             ?       30           
  Lines             ?     3997           
  Branches          ?        0           
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  Hits              ?     3947           
  Misses            ?       50           
  Partials          ?        0

Impacted Files	Coverage Δ
R/mcmc-traces.R	95.53% <100%> (ø)

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[jgabry]

Thanks for the PR! Just a few small changes requested (see comments). Also, can you add an entry to the NEWS file?