All configuration fields now have fallback values which will be used if
the field is not present. This includes mouse, key bindings and platform
specific differences.
The mouse and key bindings are now filled by default, if the user
rebinds a default mapping, it will be overwritten. To unbind a default
binding, it can be mapped to `chars: ""`.
Since all platform differences can now be correctly handled by the
`src/config/mod.rs` code, it's no longer necessary to maintain separate
configuration files, so the `alacritty_macos.yml` and
`alacritty_windows.yml` have been deleted.
Fixes#40.
Fixes#1923.
