Parse user-defined environments

[make-github-pseudonymous-again]

Fixes #17 and fixes #18.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   99.56%   99.61%   +0.05%     
==========================================
  Files           8        8              
  Lines         231      262      +31     
==========================================
+ Hits          230      261      +31     
  Misses          1        1

Impacted Files	Coverage Δ
src/grammar.js	100% <ø> (ø)	⬆️
src/tokens.js	100% <100%> (ø)	⬆️
src/shaketape.js	100% <100%> (ø)	⬆️
src/shaker.js	99.39% <100%> (+0.29%)	⬆️

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