Add build_rule to builder

[yordis]

fixes #172

[doughsay]

thanks!

[codecov]

Codecov Report

Merging #174 (1b407e0) into main (bad21b5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #174   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines          369       371    +2     
=========================================
+ Hits           369       371    +2     

Impacted Files	Coverage Δ
lib/cocktail/builder/i_calendar.ex	100.00% <100.00%> (ø)
lib/cocktail/schedule.ex	100.00% <100.00%> (ø)

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[yordis]

Sorry, I forgot to add the test case .... my bad

[yordis]

@doughsay any opportunity to merge and release this?