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%)	:arrow_down:
...ain/java/com/jsoniter/spi/UnwrapperDescriptor.java	75.86% <80%> (+13.36%)	:arrow_up:
...a/com/jsoniter/output/ReflectionObjectEncoder.java	71.11% <94.28%> (+4.02%)	:arrow_up:
...in/java/com/jsoniter/output/CodegenImplObject.java	95.65% <95%> (+0.55%)	:arrow_up:

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