Use custom sound files

[GrimPixel]

Is your feature request related to a problem? Please describe.
The sound is not appealing to all.

Describe the solution you'd like
Have the ability to change it.

[sekwah41]

Links in with #400 though would be a good idea

[Exponent4806]

A custom sound might increase significantly our motivation to work.

For example, I would like to play the sounds of "Rocky Balboa" when starting or finishing a Pomodoro...!

That helps to work harder...!

Please consider implementing a custom sound at the beginning and another at the end of each Pomodoro.

[sekwah41]

I think we need to switch the sounds generally at some point, though we also need to allow for custom sounds to be selected.