It can play sounds now

Back when I first made this, someone suggested I add a feature to make the system actually play the sounds. I said I’d do it, and then proceeded to not think about it again for 8 whole months.

Anyway, it can play dots and dashes now. morse_play() will take a Morse code string as input, and morse_play_text() will take a plain text string and play it back as Morse code. The sound priority can be set by the macro MORSE_CODE_SOUND_PRIORITY, and the volume can be set by MORSE_CODE_SOUND_VOLUME.