`w4 bundle` add option to customize keybindings
It would be great if one could customize the emulator keymap when bundling. A possible solution would be to pass a JSON config via a `--keybdings` option. The file could look something like this:

```json
{
    "player_1": {
        "left": "A",
        "down": "S",
        "up": "W",
        "right": "D",
        "button_1": "E",
        "button_2": "R",
    },
    "reboot": "B"
}
```
Fields left unchanged would default to their regular key.

### Key Format
Another thing to consider is the key format, so that both the native and the web runtime can understand it.