feat: add debug.deep and debug.shallow (#37)

<!-- Please provide enough information so that others can review your pull request. -->
<!-- Keep pull requests small and focused on a single change. -->

### Summary

Adds API proposed by https://github.com/callstack/react-native-testing-library/pull/33#issuecomment-430302079.
Also component output is now highlighted:

<img width="259" alt="screenshot 2018-10-16 at 19 33 25" src="https://user-images.githubusercontent.com/5106466/47035614-c10efb80-d17a-11e8-99ac-00f648880008.png">


### Test plan

Added tests for `debug`.

README.md

@@ -273,12 +273,52 @@ Log prettified shallowly rendered component or test instance (just like snapshot
 import { debug } from 'react-native-testing-library';
 
 debug(<Component />);
-// console.log:
-// <View>
-//   <Text>Component</Text>
-// </View>
+debug.shallow(<Component />); // an alias for `debug`
 ```
 
+logs:
+
+```jsx
+<TouchableOpacity
+  activeOpacity={0.2}
+  onPress={[Function bound fn]}
+>
+  <TextComponent
+    text="Press me"
+  />
+</TouchableOpacity>
+```
+
+There's also `debug.deep` that renders deeply to stdout.
+
+```jsx
+import { debug } from 'react-native-testing-library';
+
+debug.deep(<Component />);
+```
+
+logs:
+
+```jsx
+<View
+  accessible={true}
+  isTVSelectable={true}
+  onResponderGrant={[Function bound touchableHandleResponderGrant]}
+  // ... more props
+  style={
+    Object {
+      \\"opacity\\": 1,
+    }
+  }
+>
+  <Text>
+    Press me
+  </Text>
+</View>
+```
+
+Optionally you can provide a string message as a second argument to `debug`, which will be displayed right after the component.
+
 ## `flushMicrotasksQueue`
 
 Wait for microtasks queue to flush. Useful if you want to wait for some promises with `async/await`.