drawerLockMode (#1377)

* added drawerLockMode with cabilities to update it on the fly

* fixed incorrect name on markdown for usage

* added handling if screenProps is not being used

* Fix linting error

* Use drawerLockMode instead of lockMode

* Correct docs

* Fix flow issues

* Make drawerLockMode optional

docs/api/navigators/DrawerNavigator.md

@@ -87,7 +87,6 @@ The route configs object is a mapping from route name to a route config, which t
 
 
 ### DrawerNavigatorConfig
-
 - `drawerWidth` - Width of the drawer
 - `drawerPosition` - Options are `left` or `right`. Default is `left` position.
 - `contentComponent` - Component used to render the content of the drawer, for example, navigation items. Receives the `navigation` prop for the drawer. Defaults to `DrawerItems`. For more information, see below.
@@ -172,6 +171,10 @@ String, React Element or a function that given `{ focused: boolean, tintColor: s
 
 React Element or a function, that given `{ focused: boolean, tintColor: string }` returns a React.Element, to display in drawer sidebar
 
+#### `drawerLockMode`
+
+Specifies the [lock mode](https://facebook.github.io/react-native/docs/drawerlayoutandroid.html#drawerlockmode) of the drawer. This can also update dynamically by using screenProps.lockMode on your top level router.
+
 ### Navigator Props
 
 The navigator component created by `DrawerNavigator(...)` takes the following props: