Improve auto-generated spelling.R file (and use it)

[kbenoit]

I thought it ironic that the author of lintr would auto-generate a file (tests/spelling.R) that would fail a lint check, and also that the package itself fails its own spelling check! 😉

Great package!!

[codecov-io]

Codecov Report

Merging #18 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #18   +/-   ##
=======================================
  Coverage   42.95%   42.95%           
=======================================
  Files           7        7           
  Lines         298      298           
=======================================
  Hits          128      128           
  Misses        170      170

Impacted Files	Coverage Δ
R/spell-check.R	30.82% <0%> (ø)	⬆️

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