1024 Add Concat function

[dantownsend]

Resolves #1024

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.41%. Comparing base (70dac99) to head (6b3ba49).
Report is 7 commits behind head on master.

Files	Patch %	Lines
piccolo/columns/column_types.py	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1025      +/-   ##
==========================================
- Coverage   89.27%   88.41%   -0.86%     
==========================================
  Files         114      115       +1     
  Lines        8333     8424      +91     
==========================================
+ Hits         7439     7448       +9     
- Misses        894      976      +82     

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