Support FT.TAGVALS command

[dengliming]

A PR for #52

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   72.52%   72.65%   +0.12%     
==========================================
  Files          12       12              
  Lines        1059     1064       +5     
==========================================
+ Hits          768      773       +5     
  Misses        232      232              
  Partials       59       59              

Impacted Files	Coverage Δ
redisearch/client.go	79.44% <100.00%> (+0.32%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 910ef02...531d81d. Read the comment docs.

[filipecosta90]

thanks @dengliming . merging