Add processing stage for parallel branch coordinates

[noahrhodes]

TODO

• [x] Update documentation for :parallel_edge_offest kwarg
• [x] Add support for other edge types (dclines, anything else?)
• [x] Add tests that use parallel lines to plot

[codecov-commenter]

Codecov Report

Merging #72 (0de0a91) into master (4b89558) will increase coverage by 0.35%. The diff coverage is 100.00%.

:exclamation: Current head 0de0a91 differs from pull request most recent head eabe243. Consider uploading reports for the commit eabe243 to get more accurate results

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   93.54%   93.90%   +0.35%     
==========================================
  Files          13       14       +1     
  Lines         666      705      +39     
==========================================
+ Hits          623      662      +39     
  Misses         43       43              

Impacted Files	Coverage Δ
src/core/options.jl	100.00% <ø> (ø)
src/core/data.jl	100.00% <100.00%> (ø)
src/plots/plot.jl	96.36% <100.00%> (+0.09%)	:arrow_up:

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