Support xaringan

[lorenzwalthert]

Closes #699 and maybe even a little speed-up.

• cache_applying: 0.03 -> 0.03 (-8%)
• cache_recording: 1.04 -> 1.02 (-1.7%)
• without_cache: 2.93 -> 2.99 (1.8%)

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• cache_applying: 0.04 -> 0.04 (-6.2%)
• cache_recording: 1.37 -> 1.36 (-1.1%)
• without_cache: 4.08 -> 4.02 (-1.3%)

[codecov-io]

Codecov Report

Merging #700 (5c62494) into master (f4c395e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
+ Coverage   90.42%   90.43%   +0.01%     
==========================================
  Files          47       47              
  Lines        2245     2248       +3     
==========================================
+ Hits         2030     2033       +3     
  Misses        215      215              

Impacted Files	Coverage Δ
R/expr-is.R	88.09% <100.00%> (ø)
R/rules-spacing.R	98.46% <100.00%> (+0.02%)	⬆️

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