Add Line and LineLoop and make constructing them easier

[rdeits]

Fixes #87

[codecov-io]

Codecov Report

Merging #90 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   70.05%   70.14%   +0.08%     
==========================================
  Files          12       12              
  Lines         334      335       +1     
==========================================
+ Hits          234      235       +1     
  Misses        100      100

Impacted Files	Coverage Δ
src/MeshCat.jl	28% <ø> (ø)	⬆️
src/objects.jl	96.15% <100%> (+0.15%)	⬆️

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