markdown: Disable Codehilite language guessing

It appears that for random text the MIME lexer wins:

    In [4]: import pygments.lexer
    In [5]: pygments.lexers.guess_lexer("Hallo")
    Out[5]: <pygments.lexers.MIMELexer>

which usually generates errors for random text:

    In [11]: from markdown.extensions.codehilite import CodeHilite
    In [12]: CodeHilite(src = 'Hallo').hilite()
    Out[12]: '<div class="codehilite"><pre><span></span><span class="err">Hallo</span>\n</pre></div>\n'

which just look ugly (red border on each line).

So switch off language guessing. In that case, code blocks without a
explicit language are left as is:

    In [13]: CodeHilite(src = 'Hallo',guess_lang=False).hilite()
    Out[13]: '<div class="codehilite"><pre><span></span>Hallo\n</pre></div>\n'

In Markdown, the language should be set explicitly, for language
specific highlighting.

    :::python
    print("Hallo")

mpicms/base/templatetags/md_tags.py

@@ -19,7 +19,7 @@ def markdown(text):
         md_processor = md_module.Markdown(
             extensions=[
                 ExtraExtension(),
-                CodeHiliteExtension(),
+                CodeHiliteExtension(guess_lang=False),
                 Nl2BrExtension(),
                 SaneListExtension(),
                 SmartyExtension(smart_angled_quotes=True),