Add key_builder param to caches to customize keys

[argaen]

Closes #308

[codecov-io]

Codecov Report

Merging #310 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #310      +/-   ##
=========================================
+ Coverage    99.2%   99.2%   +<.01%     
=========================================
  Files          11      11              
  Lines         883     884       +1     
  Branches       93      93              
=========================================
+ Hits          876     877       +1     
  Misses          7       7

Impacted Files	Coverage Δ
aiocache/base.py	100% <100%> (ø)	⬆️

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