Accept mdown file extension for Markdown files.

This extension is sometimes used for Markdown files and is the default
for a few editors, such as Sublime Text.

pelican/tests/content/article_with_mdown_extension.mdown

@@ -0,0 +1,10 @@
+title: Test mdown File
+category: test
+
+Test Markdown File Header
+=========================
+
+Used for pelican test
+---------------------
+
+This is another markdown test file.  Uses the mdown extension.