Abbreviations are destroyed by line breaks

[cabo]

Input:

*[foo bar]: baz

hello foo bar babble.

hello foo
bar babble.

Output:

<p>hello <abbr title="baz">foo bar</abbr> babble.</p>

<p>hello foo
bar babble.</p>

This is bad if the markdown author uses a line-breaking editor.

[cabo]

The abbreviation scanner also doesn't like non-breaking spaces inside the abbreviation.

It should accept any kind of whitespace.