Add React.Element support for TabBar/Drawer and fix docs (#451)

* Initial impl

* Fix up some docs and support more for drawer

* Fix comments

* Support TabBar

* Make flow more correct

* Clarify even more

* Rename all the things lol

* Also rename renderLabel to getLabel

docs/api/navigators/DrawerNavigator.md

@@ -117,6 +117,34 @@ contentOptions: {
 }
 ```
 
+### Screen Navigation Options
+
+Usually you define static `navigationOptions` on your screen component. For example:
+
+```jsx
+class ProfileScreen extends React.Component {
+
+  static navigationOptions = {
+
+    title: ({ state }) => `${state.params.name}'s Profile!`,
+
+    drawer: {
+      icon: (
+        <Image src={require('./my-icon.png')} />
+      ),
+    },
+  };
+  ...
+```
+
+All `navigationOptions` for the `DrawerNavigator`:
+
+- `title` - a title (string) of the scene
+- `drawer` - a config object for the drawer:
+  - `label` - String, React Element or a function that given `{ focused: boolean, tintColor: string }` returns a React.Element, to display in drawer sidebar. When undefined, scene `title` is used
+  - `icon` - React Element or a function, that given `{ focused: boolean, tintColor: string }` returns a React.Element, to display in drawer sidebar
+
+
 ### Navigator Props
 
 The navigator component created by `DrawerNavigator(...)` takes the following props:

docs/api/navigators/StackNavigator.md

@@ -116,14 +116,14 @@ class ProfileScreen extends React.Component {
 
 All `navigationOptions` for the `StackNavigator`:
 
-- `title` - a title (string) displayed in the header
+- `title` - a title (string) of the scene
 - `header` - a config object for the header bar:
   - `visible` - Boolean toggle of header visibility. Only works when `headerMode` is `screen`.
-  - `title` - Title string used by the navigation bar, or a custom React component
-  - `backTitle` - Title string used by the back button or `null` to disable label. Defaults to `title` value by default
-  - `right` - Custom React Element to display on the right side of the header
-  - `left` - Custom React Element to display on the left side of the header
-  - `style` - Style object for the navigation bar
+  - `title` - String or React Element used by the header. Defaults to scene `title`
+  - `backTitle` - Title string used by the back button on iOS or `null` to disable label. Defaults to scene `title`
+  - `right` - React Element to display on the right side of the header
+  - `left` - React Element to display on the left side of the header
+  - `style` - Style object for the header
   - `titleStyle` - Style object for the title component
   - `tintColor` - Tint color for the header
 - `cardStack` - a config object for the card stack:

docs/api/navigators/TabNavigator.md

@@ -149,6 +149,34 @@ tabBarOptions: {
 }
 ```
 
+### Screen Navigation Options
+
+Usually you define static `navigationOptions` on your screen component. For example:
+
+```jsx
+class ProfileScreen extends React.Component {
+
+  static navigationOptions = {
+
+    title: ({ state }) => `${state.params.name}'s Profile!`,
+
+    tabBar: ({ state, setParams }) => ({
+      icon: (
+        <Image src={require('./my-icon.png')} />
+      ),
+    }),
+  };
+  ...
+```
+
+All `navigationOptions` for the `TabNavigator`:
+
+- `title` - a title (string) of the scene
+- `tabBar` - a config object for the tab bar:
+  - `visible` - Boolean toggle of tab bar visibility
+  - `icon` - React Element or a function that given `{ focused: boolean, tintColor: string }` returns a React.Element, to display in tab bar
+  - `label` - Title string of a tab displayed in the tab bar. When undefined, scene `title` is used. To hide, see `tabBarOptions.showLabel` in the previous section
+  
 ### Navigator Props
 
 The navigator component created by `TabNavigator(...)` takes the following props: