Add redis hexists command

[cep-ter]

No description provided.

[codecov-io]

Codecov Report

Merging #22 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   92.82%   92.85%   +0.03%     
==========================================
  Files           3        3              
  Lines         195      196       +1     
==========================================
+ Hits          181      182       +1     
  Misses         12       12              
  Partials        2        2              

Impacted Files	Coverage Δ
commander/commander.go	96.50% <100.00%> (+0.02%)	⬆️

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[rezkam]

@cep-ter I think we can add most of the Redis commands like this; what do you think?