Remove non-HTML support

[gucong3000]

Ref: stylelint/stylelint#3248

[codecov]

Codecov Report

Merging #43 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #43    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          13     10     -3     
  Lines         305    204   -101     
======================================
- Hits          305    204   -101

Impacted Files	Coverage Δ
lib/parse-style.js	100% <ø> (ø)	⬆️
lib/parser.js	100% <100%> (ø)	⬆️
lib/html-parser.js	100% <100%> (ø)	⬆️

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