reduce allocs and improve the render WriteString

[imxyb]

bencmp result

benchmark                     old ns/op     new ns/op     delta
BenchmarkReader_Render-12     581           545           -6.20%

benchmark                     old allocs     new allocs     delta
BenchmarkReader_Render-12     9              8              -11.11%

benchmark                     old bytes     new bytes     delta
BenchmarkReader_Render-12     1008          992           -1.59%

[codecov]

Codecov Report

Merging #2508 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2508   +/-   ##
=======================================
  Coverage   98.46%   98.46%           
=======================================
  Files          41       41           
  Lines        1951     1951           
=======================================
  Hits         1921     1921           
  Misses         17       17           
  Partials       13       13           

Impacted Files	Coverage Δ
render/text.go	100.00% <100.00%> (ø)

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[imxyb]

@appleboy PTAL, thanks

[thinkerou]

LGTM