856 Be able to extract parts of timestamps / dates

[dantownsend]

Supersedes: #857

Resolves: #856

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.89189% with 3 lines in your changes missing coverage. Please review.

Project coverage is 92.68%. Comparing base (70dac99) to head (bdc92d3).
Report is 5 commits behind head on master.

Files	Patch %	Lines
piccolo/query/functions/datetime.py	91.66%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1023      +/-   ##
==========================================
+ Coverage   89.27%   92.68%   +3.41%     
==========================================
  Files         114      115       +1     
  Lines        8333     8421      +88     
==========================================
+ Hits         7439     7805     +366     
+ Misses        894      616     -278     

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