fix(subParsers/stripLinkDefinitions): fix title attribute in link definitions

According to spec, the title attribute in link definitions can be wrapped in single quotes. Previously, showdown didn't support this.
Now the title attribute can be wrapped in single quotes.

dist/showdown.js

@@ -1905,7 +1905,7 @@ showdown.subParser('stripBlankLines', function (text) {
 showdown.subParser('stripLinkDefinitions', function (text, options, globals) {
   'use strict';
 
-  var regex = /^[ ]{0,3}\[(.+)]:[ \t]*\n?[ \t]*<?(\S+?)>?[ \t]*\n?[ \t]*(?:(\n*)["(](.+?)[")][ \t]*)?(?:\n+|(?=~0))/gm;
+  var regex = /^[ ]{0,3}\[(.+)]:[ \t]*\n?[ \t]*<?(\S+?)>?[ \t]*\n?[ \t]*(?:(\n*)["|'(](.+?)["|')][ \t]*)?(?:\n+|(?=~0))/gm;
 
   // attacklab: sentinel workarounds for lack of \A and \Z, safari\khtml bug
   text += '~0';
@@ -1919,7 +1919,7 @@ showdown.subParser('stripLinkDefinitions', function (text, options, globals) {
       return m3 + m4;
 
     } else if (m4) {
-      globals.gTitles[m1] = m4.replace(/"/g, '&quot;');
+      globals.gTitles[m1] = m4.replace(/"|'/g, '&quot;');
     }
 
     // Completely remove the definition from the text

src/subParsers/stripLinkDefinitions.js

@@ -26,7 +26,7 @@
 showdown.subParser('stripLinkDefinitions', function (text, options, globals) {
   'use strict';
 
-  var regex = /^[ ]{0,3}\[(.+)]:[ \t]*\n?[ \t]*<?(\S+?)>?[ \t]*\n?[ \t]*(?:(\n*)["(](.+?)[")][ \t]*)?(?:\n+|(?=~0))/gm;
+  var regex = /^[ ]{0,3}\[(.+)]:[ \t]*\n?[ \t]*<?(\S+?)>?[ \t]*\n?[ \t]*(?:(\n*)["|'(](.+?)["|')][ \t]*)?(?:\n+|(?=~0))/gm;
 
   // attacklab: sentinel workarounds for lack of \A and \Z, safari\khtml bug
   text += '~0';
@@ -40,7 +40,7 @@ showdown.subParser('stripLinkDefinitions', function (text, options, globals) {
       return m3 + m4;
 
     } else if (m4) {
-      globals.gTitles[m1] = m4.replace(/"/g, '&quot;');
+      globals.gTitles[m1] = m4.replace(/"|'/g, '&quot;');
     }
 
     // Completely remove the definition from the text