feat: time dependency, add clock interface

[janrnc]

Addresses #37

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.43%. Comparing base (3a11702) to head (e50398b).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
option.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   94.27%   94.43%   +0.15%     
==========================================
  Files          17       18       +1     
  Lines         804      826      +22     
==========================================
+ Hits          758      780      +22     
+ Misses         33       32       -1     
- Partials       13       14       +1     

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[github-actions]

🎉 This PR is included in version 0.6.0 🎉

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[janrnc]

See robfig/cron#327, robfig/cron#496