setprop! calls now persist across new connections

[rdeits]

On top of #141

Fixes #35

[codecov-io]

Codecov Report

Merging #142 into master will increase coverage by 2.2%.
The diff coverage is 57.14%.

@@            Coverage Diff            @@
##           master     #142     +/-   ##
=========================================
+ Coverage   79.27%   81.47%   +2.2%     
=========================================
  Files          15       18      +3     
  Lines         468      475      +7     
=========================================
+ Hits          371      387     +16     
+ Misses         97       88      -9

Impacted Files	Coverage Δ
src/lowering.jl	98.48% <ø> (+8.2%)	⬆️
src/commands.jl	83.33% <ø> (+11.9%)	⬆️
src/assets.jl	0% <0%> (ø)
src/servers.jl	4% <0%> (-0.55%)	⬇️
src/integrations.jl	100% <100%> (ø)
src/trees.jl	92.59% <100%> (ø)	⬆️
src/MeshCat.jl	80% <100%> (+46.66%)	⬆️
src/ijulia.jl	33.33% <33.33%> (ø)
src/visualizer.jl	91.66% <91.3%> (+15.61%)	⬆️
... and 2 more

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