Markdown: Add Admonition

Add Admonition extension which allows to add class attributes and a
title to a block. So

    !!!alert "Red Alert"
        Warp core breach

becomes

    <div class="admonition alert">
      <p class="admonition-title">Red Alert</p>
      <p>Warp core breach</p>
    </div>

mpicms/base/templatetags/md_tags.py

@@ -4,6 +4,7 @@
 from markdown.extensions.nl2br import Nl2BrExtension
 from markdown.extensions.sane_lists import SaneListExtension
 from markdown.extensions.smarty import SmartyExtension
+from markdown.extensions.admonition import AdmonitionExtension
 from markdown.extensions.toc import TocExtension
 
 from django import template
@@ -23,6 +24,7 @@ def markdown(text):
                 Nl2BrExtension(),
                 SaneListExtension(),
                 SmartyExtension(smart_angled_quotes=True),
+                AdmonitionExtension(),
                 TocExtension(),
             ]
         )