use headless chrome instead of jsdom

[philschatz]

This results in a significant performance improvement.

A 6 MB HTML file used to take 10+ minutes and now it only takes 2 on a Macbook Air

TODO

• [x] get console colors back (force chalk to send escape codes or render the messages in the node client; would need a colorless fallback when running in a browser)
• [x] get sourcemaps for the HTML working again (need to load jsdom and record all the selector-to-lin/col-number mappings)
• [x] get progress bar working again
  • maybe this isn't a concern anymore because the Transform is so fast

[codecov[bot]]

Codecov Report

Merging #3 into master will decrease coverage by 0.15%. The diff coverage is 83.85%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
- Coverage   88.39%   88.23%   -0.16%     
==========================================
  Files          45       73      +28     
  Lines        1611     1981     +370     
  Branches      292      353      +61     
==========================================
+ Hits         1424     1748     +324     
- Misses        187      233      +46

Impacted Files	Coverage Δ
test/unit/for-each.in.xhtml	100% <ø> (ø)
test/example/glossary.in.xhtml	100% <ø> (ø)
test/unit/has.in.xhtml	100% <ø> (ø)
test/example/exercise-numbering.in.xhtml	100% <ø> (ø)
test/unit/inside.in.xhtml	100% <ø> (ø)
test/unit/ancestor-context.in.xhtml	100% <ø> (ø)
test/example/exercise-numbering-advanced.in.xhtml	100% <ø> (ø)
test/unit/class.in.xhtml	100% <ø> (ø)
test/unit/functions.in.xhtml	100% <ø> (ø)
test/unit/namespace-attributes.in.xhtml	100% <ø> (ø)
... and 60 more

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