message: support alt+enter on mac, like slate composer
diff: @@ -186,7 +186,7 @@ export default class BasicMessageEditor extends React.Component {
  }
  handled = true;
  // insert newline on Shift+Enter
  - } else if (event.shiftKey && event.key === "Enter") {
  + } else if (event.key === "Enter" && (event.shiftKey || (IS_MAC && event.altKey))) {
  this._insertText("\n");
  handled = true;
  // autocomplete or enter to send below shouldn't have any modifier keys pressed.
  