Trigger an event when focus changes

I imagine it would be something like:

const activeWin  = require('active-win');

activeWin.on('change', win => {
    console.log(win.title);
});

With win being the same as activeWin() returns.

No idea how to achieve this though as it would have to communicate with the binary somehow. Maybe spawn the native binary when someone subscribes to the events, and have the binary push to stdout for each event.

PR welcome 😀