Added unit test to test the markdown file extension.

tests/content/article_with_markdown_extension.markdown

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

tests/test_readers.py

@@ -143,6 +143,17 @@ class MdReaderTest(unittest.TestCase):
         
         self.assertEqual(content, expected)
 
+    @unittest.skipUnless(readers.Markdown, "markdown isn't installed")
+    def test_article_with_mkd_extension(self):
+        # test to ensure the markdown extension is being processed by the correct reader
+        reader = readers.MarkdownReader({})
+        content, metadata = reader.read(_filename('article_with_markdown_extension.markdown'))
+        expected = "<h1>Test Markdown File Header</h1>\n"\
+                "<h2>Used for pelican test</h2>\n"\
+                "<p>This is another markdown test file.  Uses the markdown extension.</p>"
+        
+        self.assertEqual(content, expected)
+
     @unittest.skipUnless(readers.Markdown, "markdown isn't installed")
     def test_article_with_markdown_markup_extension(self):
         # test to ensure the markdown markup extension is being processed as expected