Try to compute game's walkthrough if it was not store in the metadata.

[MarcCote]

This PR adds the Game.walkthrough property. It first looks for a walkthrough stored in the metadata, otherwise, it tries to derive one from Game.quests. It will return None if it can find any walkthroughs.

Fixes #224

[codecov-io]

Codecov Report

Merging #225 into master will decrease coverage by 0.1%.
The diff coverage is 18.18%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   82.97%   82.86%   -0.11%     
==========================================
  Files          56       56              
  Lines        6930     6941      +11     
==========================================
+ Hits         5750     5752       +2     
- Misses       1180     1189       +9

Impacted Files	Coverage Δ
textworld/generator/game.py	94.52% <18.18%> (-1.47%)	⬇️

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