Support tabs on MarshalIndent

[shdunning]

Fixes #253

Support for using tabs on calls to MarshalIndent like so:

json.MarshalIndent(v, "", "\t")

Todo:

• error check that mixed tabs and spaces are not supported
• tests

[codecov]

Codecov Report

Merging #273 into master will decrease coverage by 0.16%.
The diff coverage is 53.84%.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
- Coverage   81.35%   81.19%   -0.17%     
==========================================
  Files          41       41              
  Lines        4978     4988      +10     
==========================================
  Hits         4050     4050              
- Misses        811      819       +8     
- Partials      117      119       +2

Impacted Files	Coverage Δ
stream.go	84.46% <50%> (-1.54%)	⬇️
config.go	88.02% <55.55%> (-0.63%)	⬇️
reflect_struct_decoder.go	44.63% <0%> (-0.84%)	⬇️

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[shdunning]

@taowen can you take a look at this one plz?

[akyoto]

I stumbled upon this problem as well, the use of tabs caused a panic on my webserver after switching to this library.
Would love it if tabs were supported.