Empty string title breaks reference style link

[justmytwospence]

# works right:
[Google]: https://google.com/ "This is a link to Google"

# broken:
[Google]: https://google.com/ ""

The reason this is an issue is that some programs (Emacs' markdown-mode in this case) auto generate reference links with empty strings like this if they aren't explicitly given a title. I debated making an issue for markdown-mode or making an issue here, but it seems like this is an edge case that should not break the rendering of the markdown, regardless.

[balinterdi]

To me, reference style links do not work at all (although I use redcarpet through middleman, a static site generator).

If I have the following

The component I described in [the intro post] [intropost]

And then the reference definition at the bottom of the post:

[intropost]: /2015/09/10/complex-component-design-in-ember-intro.html

The output is simply the input text, as if it had been parsed.