media files not suitable for prefix with _ underscore

[shanehh]

WordQuery/src/service/youdao.py

Line 149 in 99f6377

filename = u'_youdao_{}_uk.mp3'.format(self.word)

ref: https://apps.ankiweb.net/docs/manual20.html

Rename the file, adding an underscore at the start, so it becomes like "_arial.ttf". Adding an underscore will tell Anki that this file will be used on a template, and should not be deleted when checking for unused media.