[add] Switch component

'Switch' on Web can support custom sizes and colors. To do so,
Web-specific propTypes are introduced: `trackColor`, `thumbColor`,
`activeTrackColor`, and `activeThumbColor`.

docs/components/Switch.md

@@ -0,0 +1,76 @@
+# Switch
+
+This is a controlled component that requires an `onValueChange` callback that
+updates the value prop in order for the component to reflect user actions. If
+the `value` prop is not updated, the component will continue to render the
+supplied `value` prop instead of the expected result of any user actions.
+
+## Props
+
+[...View props](./View.md)
+
+**disabled**: bool = false
+
+If `true` the user won't be able to interact with the switch.
+
+**onValueChange**: func
+
+Invoked with the new value when the value changes.
+
+**value**: bool = false
+
+The value of the switch. If `true` the switch will be turned on.
+
+(web) **activeThumbColor**: color = #009688
+
+The color of the thumb grip when the switch is turned on.
+
+(web) **activeTrackColor**: color = #A3D3CF
+
+The color of the track when the switch is turned on.
+
+(web) **thumbColor**: color = #FAFAFA
+
+The color of the thumb grip when the switch is turned off.
+
+(web) **trackColor**: color = #939393
+
+The color of the track when the switch is turned off.
+
+## Examples
+
+```js
+import React, { Component } from 'react'
+import { Switch, View } from 'react-native'
+
+class ColorSwitchExample extends Component {
+  constructor(props) {
+    super(props)
+    this.state = {
+      colorTrueSwitchIsOn: true,
+      colorFalseSwitchIsOn: false
+    }
+  }
+
+  render() {
+    return (
+      <View>
+        <Switch
+          activeThumbColor='#428BCA'
+          activeTrackColor='#A0C4E3'
+          onValueChange={(value) => this.setState({ colorFalseSwitchIsOn: value })}
+          value={this.state.colorFalseSwitchIsOn}
+        />
+        <Switch
+          activeThumbColor='#5CB85C'
+          activeTrackColor='#ADDAAD'
+          onValueChange={(value) => this.setState({ colorTrueSwitchIsOn: value })}
+          thumbColor='#EBA9A7'
+          trackColor='#D9534F'
+          value={this.state.colorTrueSwitchIsOn}
+        />
+      </View>
+    )
+  }
+}
+```