Normalise line endings, add local .gitattributes

Setting `* text=auto`...

> ... ensures that all files that Git considers to be text will have normalized
> (LF) line endings in the repository
> -- [gitattributes(5)][5]

This should prevent issues such as those discussed in #11448 from cropping up.

See also [h5bp/html5-boilerplate#7f31896][h5bp].

  [5]: http://git-scm.com/docs/gitattributes
  [h5bp]: https://github.com/h5bp/html5-boilerplate/commit/7f31896fd4462c4e50748f71b24a3fb1cd375ea1#commitcomment-525963