This PR will allow the user to have selectors like '.btn, .btn2' (like in HTML)
Need to do some more testing and see if there are no other cases where a comma is allowed in the selector.

<Alloy>
  <Window layout="vertical">
    <Button class="btn">Button 1</Button>
    <Button class="btn2">Button 2</Button>
  </Window>
</Alloy>

'.btn, .btn2': {
  backgroundColor: 'red'
}

'.btn2': {
  color: 'green'
}

The later style will override the parts of the first so

'.btn, .btn2': {
  backgroundColor: 'red',
  color: 'blue'
}

'.btn2': {
  color: 'green'
}

will only make the first button text color blue.

All tests pass: 2778 specs, 0 failures
two bigger Alloy apps still look fine, so nothing broke in existing styles.