ENH: New Event classes

[MarcCote]

This PR allows Events to be combined using boolean expressions (e.g. and/or). This is needed to make more complex quests.

This PR closes #218.

[MarcCote]

@HakiRose I've also added a bunch of tests. This PR doesn't contain your modifications for text_generation nor verb_tense.

[codecov-commenter]

Codecov Report

Merging #235 (b2bda1a) into master (fd0d419) will decrease coverage by 1.55%.
The diff coverage is 92.51%.

❗ Current head b2bda1a differs from pull request most recent head ec5185b. Consider uploading reports for the commit ec5185b to get more accurate results

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
- Coverage   83.83%   82.28%   -1.56%     
==========================================
  Files          62       57       -5     
  Lines        7976     7839     -137     
==========================================
- Hits         6687     6450     -237     
- Misses       1289     1389     +100     

Impacted Files	Coverage Δ
textworld/generator/maker.py	85.51% <77.94%> (-1.89%)	⬇️
textworld/generator/text_generation.py	84.13% <83.33%> (-0.15%)	⬇️
textworld/generator/inform7/world2inform7.py	90.04% <95.04%> (+0.01%)	⬆️
textworld/generator/game.py	92.50% <95.22%> (-2.27%)	⬇️
textworld/generator/__init__.py	95.48% <100.00%> (ø)
textworld/generator/logger.py	79.48% <100.00%> (ø)
textworld/challenges/tw_cooking/cooking.py	60.84% <0.00%> (-20.19%)	⬇️
textworld/gym/utils.py	83.33% <0.00%> (-16.67%)	⬇️
textworld/generator/data/__init__.py	78.02% <0.00%> (-1.57%)	⬇️
... and 13 more