adds stats button on home screen, and creates stats screen, length still needs to be calculated

app.js

@@ -5,6 +5,7 @@ import Calendar from './components/calendar'
 import CycleDay from './components/cycle-day'
 import Chart from './components/chart/chart'
 import Settings from './components/settings'
+import Stats from './components/stats'
 
 // this is until react native fixes this bugg, see
 // https://github.com/facebook/react-native/issues/18868#issuecomment-382671739
@@ -16,5 +17,6 @@ export default createStackNavigator({
   calendar: { screen: Calendar },
   cycleDay: { screen: CycleDay },
   chart: { screen: Chart },
-  settings: { screen: Settings }
+  settings: { screen: Settings },
+  stats: { screen: Stats}
 })