added auto_update option to Column

[dantownsend]

Fixes #668

[codecov-commenter]

Codecov Report

Merging #670 (b2c9e1e) into master (0456561) will increase coverage by 4.48%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #670      +/-   ##
==========================================
+ Coverage   86.77%   91.25%   +4.48%     
==========================================
  Files          99       99              
  Lines        7211     7231      +20     
==========================================
+ Hits         6257     6599     +342     
+ Misses        954      632     -322     

Impacted Files	Coverage Δ
piccolo/columns/base.py	94.68% <100.00%> (+0.64%)	⬆️
piccolo/table.py	96.35% <100.00%> (+0.93%)	⬆️
piccolo/apps/migrations/auto/migration_manager.py	94.64% <0.00%> (+0.35%)	⬆️
piccolo/query/methods/alter.py	90.08% <0.00%> (+0.43%)	⬆️
piccolo/query/mixins.py	94.83% <0.00%> (+1.40%)	⬆️
piccolo/utils/pydantic.py	97.95% <0.00%> (+2.04%)	⬆️
piccolo/columns/defaults/time.py	70.83% <0.00%> (+2.08%)	⬆️
piccolo/query/methods/objects.py	97.14% <0.00%> (+2.85%)	⬆️
... and 22 more

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