graph plotting

[CarloLucibello]

Introduce plotting through graphviz as Catlab.jl is doing (the code in fact is taken from there).

todo:

• add docs
• add tests

[codecov]

Codecov Report

Merging #116 (8a0d15e) into master (5e65074) will decrease coverage by 3.05%.
The diff coverage is 15.09%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
- Coverage   97.35%   94.30%   -3.06%     
==========================================
  Files          63       65       +2     
  Lines        4125     4284     +159     
==========================================
+ Hits         4016     4040      +24     
- Misses        109      244     +135     

Impacted Files	Coverage Δ
src/Erdos.jl	100.00% <ø> (ø)
src/core/interface_network.jl	100.00% <ø> (ø)
src/drawing/draw.jl	0.00% <0.00%> (ø)
src/drawing/Graphviz.jl	1.92% <1.92%> (ø)
src/core/core.jl	97.43% <100.00%> (+0.32%)	⬆️

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