feat(ghCodeBlocks): add option to disable GH codeblocks

GFM support fenced codeblocks. Showdown, since very early, adopted this too.
It is now possible to disable GFM codeblocks with the option "ghCodeBlocks" set to false.
It is enabled by default

src/showdown.js

@@ -16,7 +16,8 @@ var showdown = {},
       literalMidWordUnderscores: false,
       strikethrough:             false,
       tables:                    false,
-      tablesHeaderId:            false
+      tablesHeaderId:            false,
+      ghCodeBlocks:              true  // true due to historical reasons
     },
     globalOptions = JSON.parse(JSON.stringify(defaultOptions)); //clone default options out of laziness =P
 

src/subParsers/githubCodeBlocks.js

@@ -11,6 +11,11 @@
 showdown.subParser('githubCodeBlocks', function (text, options, globals) {
   'use strict';
 
+  // early exit if option is not enabled
+  if (!options.ghCodeBlocks) {
+    return text;
+  }
+
   text += '~0';
 
   text = text.replace(/(?:^|\n)```(.*)\n([\s\S]*?)\n```/g, function (wholeMatch, m1, m2) {

test/features/disable_gh_codeblocks.html

@@ -0,0 +1,9 @@
+<p>this is some text</p>
+
+<p><code>php
+function thisThing() {
+echo "some weird formatted code!";
+}
+</code></p>
+
+<p>some other text</p>

test/features/disable_gh_codeblocks.md

@@ -0,0 +1,9 @@
+this is some text
+
+```php
+function thisThing() {
+  echo "some weird formatted code!";
+}
+```
+
+some other text

test/node/showdown.js

@@ -27,7 +27,8 @@ describe('showdown.options', function () {
         literalMidWordUnderscores: false,
         strikethrough:             false,
         tables:                    false,
-        tablesHeaderId:            false
+        tablesHeaderId:            false,
+        ghCodeBlocks:              true
       };
       expect(showdown.getDefaultOptions()).to.be.eql(opts);
     });

test/node/testsuite.features.js

@@ -21,6 +21,8 @@ describe('makeHtml() features testsuite', function () {
       converter = new showdown.Converter({literalMidWordUnderscores: true});
     } else if (testsuite[i].name === '#164.3.strikethrough') {
       converter = new showdown.Converter({strikethrough: true});
+    }  else if (testsuite[i].name === 'disable_gh_codeblocks') {
+      converter = new showdown.Converter({ghCodeBlocks: false});
     } else {
       converter = new showdown.Converter();
     }