fix(anchors): fix issue with brackets in link URL

Closes #390

src/subParsers/stripLinkDefinitions.js

@@ -6,7 +6,7 @@
 showdown.subParser('stripLinkDefinitions', function (text, options, globals) {
   'use strict';
 
-  var regex = /^ {0,3}\[(.+)]:[ \t]*\n?[ \t]*<?(\S+?)>?(?: =([*\d]+[A-Za-z%]{0,4})x([*\d]+[A-Za-z%]{0,4}))?[ \t]*\n?[ \t]*(?:(\n*)["|'(](.+?)["|')][ \t]*)?(?:\n+|(?=¨0))/gm;
+  var regex = /^ {0,3}\[(.+)]:[ \t]*\n?[ \t]*<?([^>\s]+)>?(?: =([*\d]+[A-Za-z%]{0,4})x([*\d]+[A-Za-z%]{0,4}))?[ \t]*\n?[ \t]*(?:(\n*)["|'(](.+?)["|')][ \t]*)?(?:\n+|(?=¨0))/gm;
 
   // attacklab: sentinel workarounds for lack of \A and \Z, safari\khtml bug
   text += '¨0';