message: Added freezable prop to navigation stack
  If this is true then will use react freeze to turn off renders on all but the top scene. If Suspense isn't available then freeze isn't possible
diff: @@ -4,7 +4,7 @@ import { Crumb, State } from 'navigation';
  import { NavigationContext, AsyncStateNavigator } from 'navigation-react';
  import PopSync from './PopSync';
  import Scene from './Scene';
  -type NavigationStackProps = {stateNavigator: AsyncStateNavigator, title: (state: State, data: any) => string, crumbStyle: any, unmountStyle: any, hidesTabBar: any, sharedElement: any, renderScene: (state: State, data: any) => ReactNode};
  +type NavigationStackProps = {stateNavigator: AsyncStateNavigator, title: (state: State, data: any) => string, freezable: boolean, crumbStyle: any, unmountStyle: any, hidesTabBar: any, sharedElement: any, renderScene: (state: State, data: any) => ReactNode};
  type NavigationStackState = {stateNavigator: AsyncStateNavigator, keys: string[]};
  class NavigationStack extends React.Component<NavigationStackProps, NavigationStackState> {
  @@ -19,6 +19,7 @@ class NavigationStack extends React.Component<NavigationStackProps, NavigationSt
  this.onNavigateToTop = this.onNavigateToTop.bind(this);
  }
  static defaultProps = {
  + freezable: false,
  unmountStyle: () => null,
  crumbStyle: () => null,
  hidesTabBar: () => false,
  @@ -88,7 +89,7 @@ class NavigationStack extends React.Component<NavigationStackProps, NavigationSt
  }
  render() {
  var {keys} = this.state;
  - var {stateNavigator, unmountStyle, crumbStyle, hidesTabBar, title, renderScene} = this.props;
  + var {stateNavigator, freezable, unmountStyle, crumbStyle, hidesTabBar, title, renderScene} = this.props;
  var {crumbs, nextCrumb} = stateNavigator.stateContext;
  return (
  <NVNavigationStack
  @@ -107,6 +108,7 @@ class NavigationStack extends React.Component<NavigationStackProps, NavigationSt
  key={key}
  crumb={crumb}
  sceneKey={key}
  + freezable={freezable && typeof React.Suspense !== 'undefined'}
  unmountStyle={unmountStyle}
  crumbStyle={crumbStyle}
  hidesTabBar={hidesTabBar}
  