Fix installation of JupyterLab extension.

[twavv]

Fixes #364.

This still requires the extra step of installing nodejs in the Conda environment.

[codecov]

Codecov Report

Merging #365 into master will increase coverage by 1.83%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #365      +/-   ##
=========================================
+ Coverage   60.26%   62.1%   +1.83%     
=========================================
  Files          16      16              
  Lines         526     599      +73     
=========================================
+ Hits          317     372      +55     
- Misses        209     227      +18

Impacted Files	Coverage Δ
src/scope.jl	65.47% <ø> (+3.13%)	⬆️
src/render.jl	27.53% <ø> (ø)	⬆️
src/providers/ijulia.jl	26.66% <0%> (-13.34%)	⬇️
src/providers/blink.jl	80% <0%> (-5.72%)	⬇️
src/asset.jl	79.24% <0%> (+1.24%)	⬆️
src/messaging.jl	22.22% <0%> (+2.22%)	⬆️
src/syntax.jl	93.58% <0%> (+2.92%)	⬆️
src/node.jl	90.74% <0%> (+3.78%)	⬆️
src/WebIO.jl	77.77% <0%> (+4.44%)	⬆️
... and 3 more

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[twavv]

This branch:

• Will prompt to install JupyterLab/NodeJS if you're installing using Conda.jl and they're not available
• Removes a dependency on Compat.jl that's not relevant since we're lower bounded to Julia 0.7 (the compat issue was that socket stuff was moved out of Base from 0.6 to 0.7).
• Improves some documentation around installing the extensions