82 - arrows on powerflow plot

[bryanluu]

Added arrows on powerflow plot

• Drew a layer of arrows pointing in the direction of positive power flow at the midpoint of branches
• Added show_flow, show_flow_legend, flow_color attributes and relevant aliases
• Added documentation for the above

~[ ] Added tests~

[codecov-commenter]

Codecov Report

Merging #101 (ca193de) into master (62c3038) will increase coverage by 0.16%. The diff coverage is 97.50%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   93.90%   94.07%   +0.16%     
==========================================
  Files          14       14              
  Lines         705      742      +37     
==========================================
+ Hits          662      698      +36     
- Misses         43       44       +1     

Impacted Files	Coverage Δ
src/core/options.jl	100.00% <ø> (ø)
src/plots/plot.jl	96.04% <93.33%> (-0.32%)	:arrow_down:
src/core/attribute_validation.jl	100.00% <100.00%> (ø)
src/layouts/common.jl	96.10% <100.00%> (+1.36%)	:arrow_up:

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