Listen translations

The variable `google-translate-listen-program` determines the program to use to
listen translations. By default it is `nil` and listening function is disabled. You
must define it first then listening function will be available and you can see
"Listen" button in the buffer with translation. For example, you can use
mplayer. Make sure it is installed. For Linux/Unix just change it to "mplayer". In
case of Windows put "mplayer.exe". Of course mplayer (or mplayer.exe) needs to be
located somewhere in the PATH. If not then you need to put full path to the
program. In case of Windows (and full path) make sure that you use double slashs, for
example: "C:\\mplayer\\mplayer.exe".