fix(fenced codeblocks): add tilde as fenced code block delimiter

Closes #456

Author: tivie

dist/showdown.js

@@ -20,7 +20,7 @@ showdown.subParser('githubCodeBlocks', function (text, options, globals) {
 
   text += '¨0';
 
-  text = text.replace(/(?:^|\n)```(.*)\n([\s\S]*?)\n```/g, function (wholeMatch, language, codeblock) {
+  text = text.replace(/(?:^|\n)(```+|~~~+)([^\s`~]*)\n([\s\S]*?)\n\1/g, function (wholeMatch, delim, language, codeblock) {
     var end = (options.omitExtraWLInCodeBlocks) ? '' : '\n';
 
     // First parse the github code block

dist/showdown.js.map

@@ -6,3 +6,9 @@
 <p>And some HTML</p>
 <pre><code class="html language-html">&lt;div&gt;HTML!&lt;/div&gt;
 </code></pre>
+<p>Use more than 3 backticks</p>
+<pre><code>some code
+</code></pre>
+<p>Use tilde as delimiter</p>
+<pre><code>another piece of code
+</code></pre>

dist/showdown.min.js

@@ -12,3 +12,15 @@ And some HTML
 ```html
 <div>HTML!</div>
 ```
+
+Use more than 3 backticks
+
+`````
+some code
+`````
+
+Use tilde as delimiter
+
+~~~
+another piece of code
+~~~