Add SetCharacterVolume

This pull request allows the volume of each characters to be adjusted.

I added persistent._character_volume and SetCharacterVolume.

        def SetCharacterVolume(voice_tag, volume=None):
            """
            :doc: voice_action

            This allows the volume of each characters to be adjustment.
            If `volume` is None, this returns the value of volume of `voice_tag`.
            Otherwise, this set it to `volume`.
            
            `volume` is a number between 0.0 and 1.0, and is interpreted as a
            fraction of the mixer volume for `voice` channel.
            """