Make abbreviations in cron expressions case insensitive

[reugn]

Resolves #25.

[codecov-commenter]

Codecov Report

Merging #26 (f578ff9) into master (1c112df) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   84.13%   84.16%   +0.02%     
==========================================
  Files           6        6              
  Lines         580      581       +1     
==========================================
+ Hits          488      489       +1     
  Misses         62       62              
  Partials       30       30              

Impacted Files	Coverage Δ
quartz/cron.go	78.83% <ø> (ø)
quartz/util.go	86.41% <100.00%> (+0.16%)	⬆️

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