Support for Cron expressions

[Koed00]

No description provided.

[codecov-commenter]

Codecov Report

Merging #452 into master will increase coverage by 0.25%.
The diff coverage is 81.33%.

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
+ Coverage   90.76%   91.01%   +0.25%     
==========================================
  Files          44       45       +1     
  Lines        2934     2983      +49     
==========================================
+ Hits         2663     2715      +52     
+ Misses        271      268       -3     

Impacted Files	Coverage Δ
django_q/core_signing.py	84.61% <ø> (ø)
django_q/__init__.py	71.42% <66.66%> (-28.58%)	⬇️
django_q/tests/test_brokers.py	71.69% <71.95%> (+0.32%)	⬆️
django_q/cluster.py	91.83% <85.71%> (-0.14%)	⬇️
django_q/admin.py	98.18% <92.30%> (-1.82%)	⬇️
django_q/models.py	98.21% <92.30%> (+0.76%)	⬆️
django_q/apps.py	100.00% <100.00%> (ø)
django_q/brokers/disque.py	98.14% <100.00%> (+0.10%)	⬆️
django_q/management/commands/qmonitor.py	100.00% <100.00%> (ø)
django_q/migrations/0011_auto_20200628_1055.py	100.00% <100.00%> (ø)
... and 9 more

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