:bug: Correct tokenization of id-less reference links.

 - This is an acceptable style of id-less (or implicit-id'd) reference
   links:

```gfm
[This][] is an acceptable way to define reference links. The
reference list "id" is the inferred from the link text.

[This]: http://daringfireball.net/projects/markdown/syntax#link
```

:white_check_mark: Added and renamed tests to better describe behavior.
Merge pull request #53 from dougborg/reference-style-link

Correct tokenization of id-less reference links.