Servedocs improvement + literate jl docs

[tlienart]

See #68

• added kwargs to servedocs for extra files not in docs/src
• test for kwargs above
• add Fredrik's LiterateLive example (https://github.com/fredrikekre/LiterateLive.jl)

Should probably tag a minor release after this.

[codecov-io]

Codecov Report

Merging #71 into master will increase coverage by 0.85%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   81.49%   82.35%   +0.85%     
==========================================
  Files           5        5              
  Lines         227      238      +11     
==========================================
+ Hits          185      196      +11     
  Misses         42       42

Impacted Files	Coverage Δ
src/utils.jl	83.72% <92.3%> (+5.59%)	⬆️

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