feat: Dark Theme (#131)

* refactor: Add DarkTheme, new colors in theme & update components to use it

* chore: Remove 'console.log' statements

* chore: Change dark theme property from string to boolean

* feat: Add ability to toggle the theme from the drawer

* fix: Wrap typography example screen with 'withTheme'

* style: Update components to use correct dark theme colors

* style: Update dark theme primary color and rn-navigation toolbar now gets the color from the theme

* style: Add color prop to DrawerItem and update the example

* style: Change the unchecked color in both Checkbox and RadioButton

* chore: Add `yarn-error.log` to `.gitignore`

* chore: Use lodash instead of lodash.merge

* chore: Address PR comments

example/src/FABExample.js

@@ -1,17 +1,22 @@
 /* @flow */
 
 import React, { Component } from 'react';
+import PropTypes from 'prop-types';
 import { View, StyleSheet } from 'react-native';
-import { Colors, FAB } from 'react-native-paper';
+import { Colors, FAB, withTheme } from 'react-native-paper';
 
-export default class ButtonExample extends Component {
+class ButtonExample extends Component {
   static title = 'Floating Action Button';
+  static propTypes = {
+    theme: PropTypes.object.isRequired,
+  };
 
   _handlePress = () => {};
 
   render() {
+    const { theme: { colors: { background } } } = this.props;
     return (
-      <View style={styles.container}>
+      <View style={[styles.container, { backgroundColor: background }]}>
         <View style={styles.row}>
           <FAB
             small
@@ -43,3 +48,5 @@ const styles = StyleSheet.create({
     margin: 8,
   },
 });
+
+export default withTheme(ButtonExample);