Added ability to specify link titles to inline and reference links.

Fixed whitespace bug with reference link regex.
Updated tests.

tests/data/implicit_reference.html

@@ -1,2 +1,3 @@
 <p>an <a href="http://example.com">implicit</a> reference link</p>
-<p>an <a href="http://example.com">implicit</a> reference link with an empty link definition</p>
+<p>an <a href="http://example.com">implicit</a> reference link with an empty link definition</p>
+<p>an <a href="http://example.com" title="Example">explicit</a> reference link with a title</p>

tests/data/implicit_reference.md

@@ -2,4 +2,8 @@ an [implicit] reference link
 
 [implicit]: http://example.com
 
-an [implicit][] reference link with an empty link definition
+an [implicit][] reference link with an empty link definition
+
+an [explicit][example] reference link with a title
+
+[example]: http://example.com "Example"

tests/data/inline_link.html

@@ -1,3 +1,4 @@
 <p><a href="http://example.com">link</a></p>
 <p><a href="http://example.com"><code>link</code></a></p>
+<p><a href="http://example.com" title="Example">link with title</a></p>
 <p><a href="http://example.com"><img alt="MD Logo" src="http://parsedown.org/md.png"></a></p>

tests/data/inline_link.md

@@ -2,4 +2,6 @@
 
 [`link`](http://example.com)
 
+[link with title](http://example.com "Example")
+
 [![MD Logo](http://parsedown.org/md.png)](http://example.com)