994 Add support for more functions (upper, lower etc)

[dantownsend]

Resolves #994

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.25806% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 79.33%. Comparing base (919983a) to head (7309a7b).
Report is 6 commits behind head on master.

Files	Patch %	Lines
piccolo/querystring.py	70.58%	10 Missing ⚠️
piccolo/query/functions/base.py	66.66%	6 Missing ⚠️
piccolo/columns/m2m.py	50.00%	3 Missing ⚠️
piccolo/columns/column_types.py	85.71%	1 Missing ⚠️
piccolo/query/methods/select.py	93.75%	1 Missing ⚠️
piccolo/query/mixins.py	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #996       +/-   ##
===========================================
- Coverage   92.78%   79.33%   -13.46%     
===========================================
  Files         108      111        +3     
  Lines        8182     8255       +73     
===========================================
- Hits         7592     6549     -1043     
- Misses        590     1706     +1116     

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