[change] TextInput: implement placeholder and placeholderTextColor

Without access to the Shadow DOM pseudo-elements, the placeholder
behaviour needs to be reimplemented.

Update to match React Native's modification to `TextInput` to include
all `View` props and use the `Text` style props.

Fix #12
Fix #48

examples/components/App.js

@@ -95,10 +95,10 @@ export default class App extends React.Component {
         />
         <TextInput secureTextEntry />
         <TextInput defaultValue='read only' editable={false} />
-        <TextInput keyboardType='email-address' />
+        <TextInput keyboardType='email-address' placeholder='you@domain.com' placeholderTextColor='red' />
         <TextInput keyboardType='numeric' />
         <TextInput keyboardType='phone-pad' />
-        <TextInput keyboardType='url' selectTextOnFocus />
+        <TextInput defaultValue='https://delete-me' keyboardType='url' placeholder='https://www.some-website.com' selectTextOnFocus />
         <TextInput
           defaultValue='default value'
           maxNumberOfLines={10}

examples/components/MediaQueryWidget.js

@@ -28,7 +28,7 @@ const MediaQueryWidget = ({ mediaQuery = {} }) => {
   return (
     <View style={styles.root}>
       <Text style={styles.heading}>Active Media Query</Text>
-      <Text>{`"${active.alias}"`} {active.mql.media}</Text>
+      <Text>{`"${active.alias}"`} {active.mql && active.mql.media}</Text>
     </View>
   )
 }