Added type="button" to <button>s without any type

Created by: Herst

https://github.com/twbs/bootlint/wiki/W007

Found as part of my work on bootlint for BS4: twbs/bootlint#410.

I created the following one-liner to do it:

find docs/ -type f -exec perl -pi -e \
  's/(?<=<button )(.*?)(?=>)/@{[(index($1,"type=")!=-1?"$1":"type=\"button\" $1")]}/g' {} +

BTW, I am occasionally rebasing my patch branches so you shouldn't need to merge the current HEAD of v4-dev into my patch.