ENH: New Event classes

[Marc-Alexandre Côté (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.

[Marc-Alexandre Côté (MarcCote)]

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

[Codecov Comments Bot (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% 
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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