parse input containing NaN and Infinity values

[trinitum]

JSON standard does not allow NaN or Infinity, but some applications
produce output that contains such values. One example of such
application would be TensorFlow Serving[1]. An example of JSON parser
that accepts NaN and Infinity is the one that comes with Python[2]. This
commit introduces a new configuration parameter AllowNaN, that enables
jsoniter to unmarshal input containing NaN and Infinity.

1. https://www.tensorflow.org/tfx/serving/api_rest#json_conformance
2. https://docs.python.org/3/library/json.html#infinite-and-nan-number-values

[codecov]

Codecov Report

Merging #434 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
+ Coverage   86.46%   86.63%   +0.16%     
==========================================
  Files          41       41              
  Lines        5106     5139      +33     
==========================================
+ Hits         4415     4452      +37     
+ Misses        555      552       -3     
+ Partials      136      135       -1

Impacted Files	Coverage Δ
config.go	89.36% <ø> (ø)	⬆️
iter.go	90.18% <100%> (+0.09%)	⬆️
iter_float.go	90.53% <100%> (+2.11%)	⬆️
reflect_struct_decoder.go	81.62% <0%> (+0.26%)	⬆️

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