Add --strip-trailing-whitespace

[phadej]

@RyanGlScott I tested this locally with trifecta. I'll appreciate if you check this too before I make a release and wire new version into haskell-ci.

[RyanGlScott]

Thanks, @phadej! I've also verified that this works locally.

One naming question: should we name the flag to be the same thing that diff uses? diff has a family of flags related to whitespace:

$ diff --help
<...>
  -E, --ignore-tab-expansion      ignore changes due to tab expansion
  -Z, --ignore-trailing-space     ignore white space at line end
  -b, --ignore-space-change       ignore changes in the amount of white space
  -w, --ignore-all-space          ignore all white space
  -B, --ignore-blank-lines        ignore changes where lines are all blank

What this PR calls --strip-trailing-whitespace corresponds to diff's --ignore-trailing-space. Do you think it's worth naming it --ignore-trailing-space on the cabal-docspec side for consistency's sake?

[phadej]

@RyanGlScott good point. I changed the flag name