Add support for [code]

Add support for the [code] bbcode, making use of the new plain_text config option.

The code block is converted to be wrapped in "```" (on its own line), and crucially the block contents are unaltered by any further parsing.

TODO: In fact it will still convert ">" to ">" and other html entities, which ideally it wouldn't, but when calling with `bbcode_to_md(false)` this is disactivated.

TODO: We can pass a language to achieve syntax highlighting, such as [code=javascript] in some bbcode flavours, and in the bbcode.org reference. This could be converted to "```javascript" in some markdown flavours. Currently it will just ignore the language information and convert a plain "```" block.

lib/tags/tags.rb

@@ -20,6 +20,11 @@ module Tags
         :html_open => '~~', :html_close => '~~',
         :description => 'Strike-through text',
         :example => 'This is <s>strike-through text</s>.'},
+      :code => {
+        :html_open => "\n```\n", :html_close => "\n```\n",
+        :description => 'Code block',
+        :example => '[code]code[/code].',
+        :plain_tag => true},
       :center => {
         :html_open => '', :html_close => '',
         :description => 'Center a text',

test/ruby_bbcode_test.rb

@@ -43,6 +43,14 @@ def test_color
   def test_center
     assert_equal 'centered', '[center]centered[/center]'.bbcode_to_md
   end
+
+  def test_code
+    assert_equal "\n```\ncode\n```\n", '[code]code[/code]'.bbcode_to_md
+    assert_equal "\n```\ncode\n```\n", '[code=javascript]code[/code]'.bbcode_to_md
+    assert_equal "\n```\nSome [b]bbcode[/b] :-)\n```\n", '[code]Some [b]bbcode[/b] :-)[/code]'.bbcode_to_md
+    assert_equal "\n```\nSome &lt;b&gt;html&lt;/b&gt;\n```\n", '[code]Some <b>html</b>[/code]'.bbcode_to_md
+    assert_equal "\n```\nSome <b>html</b>\n```\n", '[code]Some <b>html</b>[/code]'.bbcode_to_md(false)
+  end
 
   def test_ordered_list
     assert_equal "\n  1. item 1\n  1. item 2\n\n", '[ol][li]item 1[/li][li]item 2[/li][/ol]'.bbcode_to_md