Support FT.SYNADD FT.SYNUPDATE FT.SYNDUMP commands

[dengliming]

Solved #42

[codecov]

Codecov Report

Merging #55 into master will increase coverage by 0.15%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   72.59%   72.74%   +0.15%     
==========================================
  Files          12       12              
  Lines        1080     1108      +28     
==========================================
+ Hits          784      806      +22     
- Misses        236      239       +3     
- Partials       60       63       +3     

Impacted Files	Coverage Δ
redisearch/client.go	79.66% <78.57%> (-0.10%)	⬇️

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[filipecosta90]

Thanks @dengliming . Merging 👍