Respect knitr chunk headers for spinning

[lorenzwalthert]

Closes #361.

[codecov-io]

Codecov Report

Merging #362 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
+ Coverage   91.46%   91.49%   +0.02%     
==========================================
  Files          30       30              
  Lines        1418     1422       +4     
==========================================
+ Hits         1297     1301       +4     
  Misses        121      121

Impacted Files	Coverage Δ
R/rules-spacing.R	96.07% <100%> (ø)	⬆️
R/expr-is.R	96.77% <100%> (+0.47%)	⬆️

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