Disable colored messages in Windows (related #794)

youtube_dl/FileDownloader.py

@@ -254,7 +254,7 @@ class FileDownloader(object):
         Print the message to stderr, it will be prefixed with 'WARNING:'
         If stderr is a tty file the 'WARNING:' will be colored
         '''
-        if sys.stderr.isatty():
+        if sys.stderr.isatty() and os.name != 'nt':
             _msg_header=u'\033[0;33mWARNING:\033[0m'
         else:
             _msg_header=u'WARNING:'
@@ -266,7 +266,7 @@ class FileDownloader(object):
         Do the same as trouble, but prefixes the message with 'ERROR:', colored
         in red if stderr is a tty file.
         '''
-        if sys.stderr.isatty():
+        if sys.stderr.isatty() and os.name != 'nt':
             _msg_header = u'\033[0;31mERROR:\033[0m'
         else:
             _msg_header = u'ERROR:'