Added MarshalBytesWithString in Config.

[haosdent]

Sometimes we need to marshal byte slice in string format, however,
we try to avoid to convert byte slice into string to avoid
allocation.

[codecov]

Codecov Report

Merging #238 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@ Coverage Diff @@ ## master #238 +/- ## ========================================== - Coverage 85.74% 85.71% -0.03%  ========================================== Files 40 40 Lines 5038 5042 +4 ========================================== + Hits 4320 4322 +2  - Misses 573 575 +2  Partials 145 145

Impacted Files	Coverage Δ
feature_config.go	79.37% <0%> (-2.04%)	⬇️
feature_reflect_struct_decoder.go	88.46% <0%> (+0.29%)	⬆️

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[taowen]

this should be done via extension.

[taowen]

see https://github.com/json-iterator/go/blob/master/extra/binary_as_string_codec_test.go

[haosdent]

Many thx!