Angle bracket enclosed URL in reference link fails.

Input:

```
[a][b]

[b]:
```

The output `href` contains `<>`, but the original docs http://daringfireball.net/projects/markdown/syntax#link say it should work:

> The link URL may, optionally, be surrounded by angle brackets

and it works on every major implementation except multi markdown.

If I could turn back time, I would not allow angle brackets like your implementation: it is better to have a single way of doing things.

But what is done is done, so until we have a markdown specification, it is better to be compatible IMHO.

Discovered with the [Markdown Test Suite](https://github.com/karlcow/markdown-testsuite) at https://github.com/karlcow/markdown-testsuite/pull/50 . Congrats: this is the only failing test!