fix(images): fix js error when using image references

In some circumstances, on a reference style image, the last capturing
group is ignored, which causes the fucntion argument to return the number
of matches instead of a string (or undefined).
Checking if the title parameter is a string ensures that the title
parameter is actually something that was caught by the regex and not some
metadata.

Closes #585

src/subParsers/images.js

@@ -61,7 +61,7 @@ showdown.subParser('images', function (text, options, globals) {
     url = url.replace(showdown.helper.regexes.asteriskDashAndColon, showdown.helper.escapeCharactersCallback);
     var result = '<img src="' + url + '" alt="' + altText + '"';
 
-    if (title) {
+    if (title && showdown.helper.isString(title)) {
       title = title
         .replace(/"/g, '&quot;')
       //title = showdown.helper.escapeCharacters(title, '*_', false);

test/functional/makehtml/cases/issues/#585.error-when-using-image-references.html

@@ -0,0 +1 @@
+<p>[<img src="http://example.com/foo.png" alt="the-image" />]</p>

test/functional/makehtml/cases/issues/#585.error-when-using-image-references.md

@@ -0,0 +1,3 @@
+[![the-image]]
+
+[the-image]: http://example.com/foo.png