accept break and spaces in a reference label

Markdown.pl 1.0.2b8 support line breaks in reference label
~~~
This one has a [line
break].

This one has a [line
break] with a line-ending space.

    [line break]: /foo
~~~
This one has a [line
break].

This one has a [line
break] with a line-ending space.

[line break]: /foo

- - -
(GFM support it correctly)

and this fix also accept spaces in label like as many other Markdown dialects such as Marked, pandoc etc.