Print longer arrays without newlines

[IanButterworth]

Saving JSON files with long arrays results in very long documents currently because of all the newlines.
This only prints newlines after array elements if the length of the array is > 5.

i.e.

{
  "x": [1,2,3,4,5,6,7,8,9,10]
}

instead of

{
  "x": [
    1,
    2,
    3,
    4,
    5,
    6,
    7,
    8,
    9,
    10
 ]
}

Fixes #291

[IanButterworth]

The travis error is just a Pkg download glitch

[codecov]

Codecov Report

Merging #305 into master will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
+ Coverage   92.60%   92.74%   +0.13%     
==========================================
  Files           6        6              
  Lines         311      317       +6     
==========================================
+ Hits          288      294       +6     
  Misses         23       23              

Impacted Files	Coverage Δ
src/Writer.jl	85.86% <100.00%> (+0.98%)	⬆️

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[IanButterworth]

This shouldn't be merged as-is. Needs rethinking.
Also JSON3.jl doesn't print newlines for arrays, so I'm giving that a go for now

[gasagna]

Any chance this can be revived?