App for native messaging

@devoncarew 

Add a initial version of the native messaging app with host in dart and app in js.
`if` and `else` branches are largely the same. I think it would be more maintainable to just have three lines like this:
```
document.getElementById('connect-button').style.display = port ? 'none' : 'block';
```

Or better yet, do all this in CSS.