Memoize Document#excerpt_separator (#7569)
Memoize Document#excerpt_separator (#7569)

Merge pull request 7569
tillgrallert/jekyll,Strangehill/jekyll,x-way/jekyll,tillgrallert/jekyll,Strangehill/jekyll,ryanshaw/jekyll,fulldecent/jekyll,chrisfinazzo/jekyll,tillgrallert/jekyll,fulldecent/jekyll,yhironaka/yhironaka.github.io,tillgrallert/jekyll,Strangehill/jekyll,fulldecent/jekyll,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,jekyll/jekyll,ryanshaw/jekyll,Strangehill/jekyll,x-way/jekyll,x-way/jekyll,fulldecent/jekyll,x-way/jekyll,ryanshaw/jekyll,Strangehill/jekyll,chrisfinazzo/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll,jekyll/jekyll,jaybe-jekyll/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll