Create app.toml with default values if it doesn't exist

[yerke]

Is your feature request related to a problem? Please describe.
I was trying to figure out how to enable media controls on macOS. It took me a while to figure out the correct syntax of app.toml by looking in examples directory.

Describe the solution you'd like
If app.toml doesn't exist in $HOME/.config/spotify-player, create it with default values, so it's easier for users to modify it.

Describe alternatives you've considered
Do nothing.

Additional context
Thanks for creating spotify-player. It seems really solid.