exporting nextToken() allows custom Unmarshal implementation

[alvarolm]

Required for a custom Unmarshal implementation to check for bytes left after unmarshaling
https://github.com/json-iterator/go/blob/master/config.go#L349

why a custom Unmarshal implementation could be needed ?
since codecs only provide unsafe.Pointer and *Iterator but not the direct value of v that may be needed to avoid reflection cost or just direct handling

thanks !

[codecov]

Codecov Report

Merging #481 into master will not change coverage.
The diff coverage is 98.50%.

@@           Coverage Diff           @@
##           master     #481   +/-   ##
=======================================
  Coverage   86.32%   86.32%           
=======================================
  Files          41       41           
  Lines        5112     5112           
=======================================
  Hits         4413     4413           
  Misses        558      558           
  Partials      141      141           

Impacted Files	Coverage Δ
iter_object.go	67.89% <95.23%> (ø)
adapter.go	90.00% <100.00%> (ø)
any.go	80.20% <100.00%> (ø)
config.go	89.36% <100.00%> (ø)
iter.go	90.09% <100.00%> (ø)
iter_array.go	76.59% <100.00%> (ø)
iter_float.go	88.41% <100.00%> (ø)
iter_int.go	90.37% <100.00%> (ø)
iter_skip.go	84.88% <100.00%> (ø)
iter_str.go	90.90% <100.00%> (ø)
... and 6 more

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[alvarolm]

cc @AllenX2018

[alvarolm]

cc @taowen

[AllenX2018]

Hi. Can you give an example when you need this function?