Add graph visualization tool.

[MarcCote]

This PR adds support for visualizing the underlying graph of a game state.

This PR also fixes #162

TODO:

• Make visualization dependencies optional.
• Add test for show_graph.

[codecov-io]

Codecov Report

Merging #189 into master will decrease coverage by <.01%.
The diff coverage is 78.06%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
- Coverage   81.01%   81.01%   -0.01%     
==========================================
  Files          56       57       +1     
  Lines        6757     6925     +168     
==========================================
+ Hits         5474     5610     +136     
- Misses       1283     1315      +32

Impacted Files	Coverage Δ
textworld/render/__init__.py	100% <100%> (ø)	⬆️
textworld/envs/wrappers/viewer.py	80.95% <50%> (+0.46%)	⬆️
textworld/render/serve.py	47.64% <63.88%> (-0.36%)	⬇️
textworld/render/render.py	60.37% <69.56%> (+3.78%)	⬆️
textworld/render/graph.py	91.66% <91.66%> (ø)

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