Add a minIterationDuration option

[na--]

This should fix #818, though I didn't have time to test it very much...

[codecov-io]

Codecov Report

Merging #821 into master will increase coverage by 0.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #821      +/-   ##
==========================================
+ Coverage    69.9%   69.92%   +0.01%     
==========================================
  Files         111      111              
  Lines        8683     8691       +8     
==========================================
+ Hits         6070     6077       +7     
  Misses       2214     2214              
- Partials      399      400       +1

Impacted Files	Coverage Δ
lib/options.go	92.5% <0%> (-1.18%)	⬇️
cmd/options.go	60.9% <100%> (+0.72%)	⬆️
js/runner.go	85.71% <100%> (+0.24%)	⬆️
core/engine.go	92.99% <0%> (ø)	⬆️
core/local/local.go	91.49% <0%> (+0.34%)	⬆️

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[mstoykov]

LGTM