Support simpleValue type for @JsonUnwrapper

[shaoxt]

The feature is similar as @jsonvalue in jackson

[codecov-io]

Codecov Report

Merging #223 into master will increase coverage by 0.15%.
The diff coverage is 92.04%.

@@ Coverage Diff @@ ## master #223 +/- ## ========================================== + Coverage 68.31% 68.47% +0.15%  ========================================== Files 107 107 Lines 7334 7371 +37 Branches 1388 1399 +11 ========================================== + Hits 5010 5047 +37  Misses 1868 1868 Partials 456 456

Impacted Files	Coverage Δ
src/main/java/com/jsoniter/spi/Config.java	84.09% <75%> (-0.22%)	⬇️
...ain/java/com/jsoniter/spi/UnwrapperDescriptor.java	75.86% <80%> (+13.36%)	⬆️
...a/com/jsoniter/output/ReflectionObjectEncoder.java	71.11% <94.28%> (+4.02%)	⬆️
...in/java/com/jsoniter/output/CodegenImplObject.java	95.65% <95%> (+0.55%)	⬆️

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