add support for returning to insert

[dantownsend]

Supersedes this PR: #203

Fixes #171

Adds support for the returning clause with Insert:

>>> await Manager.insert(
...    Manager(name="Maz"),
...    Manager(name="Guido")
... ).returning(Manager.id, Manger.name)
[{'id': 1, 'name': 'Maz'}, {'id': 2, 'name': 'Guido'}]

[codecov-commenter]

Codecov Report

Merging #564 (40e1ecc) into master (55bde6e) will decrease coverage by 0.01%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
- Coverage   90.85%   90.84%   -0.02%     
==========================================
  Files         105      105              
  Lines        7166     7174       +8     
==========================================
+ Hits         6511     6517       +6     
- Misses        655      657       +2     

Impacted Files	Coverage Δ
piccolo/querystring.py	91.13% <ø> (ø)
piccolo/engine/base.py	74.13% <50.00%> (-0.87%)	⬇️
piccolo/engine/postgres.py	91.34% <50.00%> (-0.41%)	⬇️
piccolo/engine/sqlite.py	97.02% <100.00%> (+0.02%)	⬆️
piccolo/query/methods/insert.py	100.00% <100.00%> (ø)
piccolo/query/mixins.py	95.50% <100.00%> (ø)
piccolo/table.py	95.59% <100.00%> (ø)

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