Add 'successSound' config option

[Klakurka]

The default points to the built-in sound.

I'm thinking we can allow them to set any valid path to a sound file.

If they set it to something like a blank string, false, or null then it will effectively be like muting it.

Obviously they could also add this code into the success callback so I'm also open to just doing a 'disable-sound' config option instead - but we should discuss it before developing.

[lissavxo]

I think we could support both success-sound and disable-sound props. If the provided sound link is invalid, we could fall back to the default sound or mute it. If disable-sound is true, we mute it entirely.