config.tts_function to support user defined TTS processor

This branch adds config.tts_function to support user defined TTS function.
(Related to #400)

The shortest example code:

    init -1 python hide:
        # This function write TTS texts to stdout
        def print_stdout(s):
            print s
        
        config.tts_function = print_stdout

We can change print_stdout to make variation.
Is it possible to give character names to TTS function? It becomes a hint to change the voice like a boy or a girl.