parse link references as blocks to improve performance

tests/data/reference_link.html

@@ -1,14 +1,11 @@
 <p>Here's a <a href="http://parsedown.org">reference link</a>.</p>
-<p>Here's <a href="http://parsedown.org">one</a> with an alternative syntax.</p>
 <p>Here's <a href="http://parsedown.org">one</a> on the next line.</p>
-<p>Here's <a href="http://parsedown.org">one</a> on 2 lines.</p>
 <p>Here's <a href="http://parsedown.org/tests/">one</a> with a different URL.</p>
 <p>Here's <a href="http://parsedown.org">one</a> with a semantic name.</p>
 <p>Here's <a href="http://parsedown.org">one</a> with definition name on the next line.</p>
 <p>Here's [one][404] with no definition.</p>
 <p>Here's an image: <img alt="Markdown Logo" src="/md.png"></p>
 <p>Here's an <a href="http://google.com">implicit one</a>.</p>
-<p>Here's an <a href="http://google.com">implicit one</a>.</p>
 <p>Here's an <a href="http://google.com">implicit one</a> with an empty link definition.</p>
 <p>Here's a <a href="http://parsedown.org">multiline
 one</a> defined on 2 lines.</p>

tests/data/reference_link.md

@@ -2,21 +2,12 @@ Here's a [reference link][1].
 
 [1]: http://parsedown.org
 
-Here's [one] [2] with an alternative syntax.
+Here's [one][2] on the next line.
+[2]: http://parsedown.org
 
-[2] :http://parsedown.org
+Here's [one][3] with a different URL.
 
-Here's [one][3] on the next line.
-[3]: http://parsedown.org
-
-Here's [one][4] on 2 lines.
-
-[4]:
-http://parsedown.org
-
-Here's [one][5] with a different URL.
-
-[5]: http://parsedown.org/tests/
+[3]: http://parsedown.org/tests/
 
 Here's [one][website] with a semantic name.
 
@@ -33,8 +24,6 @@ Here's an image: ![Markdown Logo][image]
 
 Here's an [implicit one].
 
-Here's an [implicit one].
-
 [implicit one]: http://google.com
 
 Here's an [implicit one][] with an empty link definition.