Disable conversion of a tags for audio files to audio tags

[MayamaTakeshi]

This will eventually cause problems with complex notes (detected while testing with https://github.com/taqueci/redmine_wysiwyg_editor).

So for now it should be disabled. Later we can reenable it but:

• it must be put inside a try/catch to prevent javascript client code to be interrupted in case of some error
• it should be configurable (on/off) via plugin settings