GFM: be more clever on parsing fenced codeblock

lib/kramdown/parser/gfm.rb

@@ -85,8 +85,8 @@ def parse_atx_header_gfm_quirk
         true
       end
 
-      FENCED_CODEBLOCK_START = /^[~`]{3,}/
-      FENCED_CODEBLOCK_MATCH = /^(([~`]){3,})\s*?((\S+?)(?:\?\S*)?)?\s*?\n(.*?)^\1\2*\s*?\n/m
+      FENCED_CODEBLOCK_START = /^[ ]{0,3}[~`]{3,}/
+      FENCED_CODEBLOCK_MATCH = /^[ ]{0,3}(([~`]){3,})\s*?((\S+?)(?:\?\S*)?)?\s*?\n(.*?)^[ ]{0,3}\1\2*\s*?\n/m
       define_parser(:codeblock_fenced_gfm, FENCED_CODEBLOCK_START, nil, 'parse_codeblock_fenced')
 
       STRIKETHROUGH_DELIM = /~~/

test/testcases_gfm/codeblock_fenced.html

@@ -0,0 +1,20 @@
+<p>normal</p>
+
+<pre><code class="language-ruby">require 'kramdown'
+
+Kramdown::Document.new(text).to_html
+</code></pre>
+
+<p>indent with tab</p>
+
+<pre><code>```ruby
+require 'kramdown'
+
+Kramdown::Document.new(text).to_html
+```
+</code></pre>
+
+<p>indent with 2 spaces</p>
+
+<pre><code class="language-js">  console.log("hello");
+</code></pre>

test/testcases_gfm/codeblock_fenced.options

@@ -0,0 +1 @@
+:enable_coderay: false

test/testcases_gfm/codeblock_fenced.text

@@ -0,0 +1,21 @@
+normal
+
+```ruby
+require 'kramdown'
+
+Kramdown::Document.new(text).to_html
+```
+
+indent with tab
+
+	```ruby
+	require 'kramdown'
+
+	Kramdown::Document.new(text).to_html
+	```
+
+indent with 2 spaces
+
+  ```js
+  console.log("hello");
+  ```