feat: Accept InnerClass (#127)

* Accept InnerClass

* Added innerClass to readme

Author: DannyAziz

README.md

@@ -54,6 +54,7 @@ import Sticky from 'react-stickynode';
 | `innerZ` | Number/String | z-index of the sticky. |
 | `enableTransforms` | Boolean | Enable the use of CSS3 transforms (true by default). |
 | `activeClass` | String | Class name to be applied to the element when the sticky state is active (`active` by default). |
+| `innerClass` | String | Class name to be applied to the inner element (`''` by default). |
 | `releasedClass` | String | Class name to be applied to the element when the sticky state is released (`released` by default). |
 | `onStateChange` | Function | Callback for when the sticky state changes. See below. |
 | `shouldFreeze` | Function | Callback to indicate when the sticky plugin should freeze position and ignore scroll/resize events. See below. |