Mass M2M join in Table object

[northpowered]

Adds in piccolo.Table:

• __join_field coroutine
• join_m2m coroutine

join_m2m() provides an ability to get M2M related lists of objects, using already defined attributes

Examples are avaliable in docstings of methods and the issue

[codecov-commenter]

Codecov Report

Merging #566 (e2a3ce5) into master (0c44bb2) will increase coverage by 0.26%. The diff coverage is 45.45%.

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@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
+ Coverage   90.82%   91.08%   +0.26%     
==========================================
  Files         105      108       +3     
  Lines        7183     7541     +358     
==========================================
+ Hits         6524     6869     +345     
- Misses        659      672      +13     

Impacted Files	Coverage Δ
piccolo/table.py	91.57% <45.45%> (-4.07%)	:arrow_down:
piccolo/apps/user/commands/create.py	38.88% <0.00%> (-9.03%)	:arrow_down:
piccolo/columns/defaults/time.py	70.83% <0.00%> (-4.78%)	:arrow_down:
piccolo/columns/defaults/timestamp.py	94.54% <0.00%> (-3.38%)	:arrow_down:
piccolo/engine/sqlite.py	93.10% <0.00%> (-3.15%)	:arrow_down:
piccolo/query/methods/insert.py	97.22% <0.00%> (-2.78%)	:arrow_down:
piccolo/apps/playground/commands/run.py	45.23% <0.00%> (-2.67%)	:arrow_down:
piccolo/query/methods/select.py	97.90% <0.00%> (-1.67%)	:arrow_down:
...ccolo/apps/migrations/auto/serialisation_legacy.py	50.00% <0.00%> (-1.52%)	:arrow_down:
... and 56 more

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[lgtm-com[bot]]

This pull request introduces 2 alerts when merging 9952e81b59010d75b206fd8afe138785d8c9039b into 0c44bb22be451e27318ddc2594e7e5a775494f80 - view on LGTM.com

new alerts:

• 2 for Testing equality to None