Make sure we don't get duplicate reference links, even with
`--reference-location=section`.

```
% pandoc --reference-links -t markdown --reference-location=section --atx-headers
# a

![](a)

# b

![](b)

^D
# a

![][1]

  [1]: a

# b

![][2]

  [2]: b
```

Subsidiary issue:  allow line break between reference link
url/title and attributes:

```
% pandoc
[a]

[a]: url
{.class}
^D
<p><a href="url" class="class">a</a></p>
```