Fixed UnicodeEncodeError when output from markdown_py is piped into a…

…nother program.

In this case text should be encoded into the output encoding explicitly, because
sys.stdout.encoding is None, when piping data.

markdown/__init__.py

@@ -373,7 +373,8 @@ def convertFile(self, input=None, output=None, encoding=None):
                 output_file.write(html)
                 # Don't close here. User may want to write more.
         else:
-            sys.stdout.write(html)
+            stdout = codecs.getwriter(encoding)(sys.stdout, errors="xmlcharrefreplace")
+            stdout.write(html)
 
         return self