PiccoloCRUD accepts visible_columns

[dantownsend]

Related to piccolo-orm/piccolo_admin#73

[codecov-commenter]

Codecov Report

Merging #96 (3d88dd5) into master (1f3ed05) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   86.01%   86.02%   +0.01%     
==========================================
  Files          27       27              
  Lines        1158     1159       +1     
==========================================
+ Hits          996      997       +1     
  Misses        162      162              

Impacted Files	Coverage Δ
piccolo_api/crud/endpoints.py	93.43% <100.00%> (+0.02%)	⬆️

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