Link labels comparison issues

[mity]

(from https://github.com/mity/md4c/pull/92#discussion_r341847612)

Input:

[ab]: /foo

[a]  
[ab] 
[abc]

Actual output:

<p><a href="/foo">a</a>
<a href="/foo">ab</a>
<a href="/foo">abc</a></p>

Expected output:

<p>[a]
<a href="/foo">ab</a>
[abc]</p>

[mity]

Another issue is with whitespace in the labels.

Input:

[a b]: /foo
[a   b]

Actual output:

<p>[a   b]</p>

Expected output:

<p><a href="/foo">a   b</a></p>

(I.e. for comparison purposes, all whitespace sequences should be considered equivalent.)