added filter columns for UI

[sinisaos]

Related to #73

[codecov-commenter]

Codecov Report

Merging #103 (f27b6ee) into master (4ce6547) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   98.32%   98.42%   +0.09%     
==========================================
  Files           3        3              
  Lines         179      190      +11     
==========================================
+ Hits          176      187      +11     
  Misses          3        3              

Impacted Files	Coverage Δ
piccolo_admin/endpoints.py	98.39% <100.00%> (+0.10%)	⬆️

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[sinisaos]

@dantownsend Can you check if everything is OK and can you add images to docs. I don't have Safari and it would be good if the images were from the same browser.

[dantownsend]

@sinisaos This looks great - thanks

[dantownsend]

@sinisaos Thanks again for this. I added the images to the docs.

I renamed the parameter from filter_columns to visible_filters - I thought it was marginally easier to remember, as it's more similar to the existing visible_columns parameter.