Export ThemeContext (#60)

We'd like to use ThemeContext in order to work with `ThemeContext.Consumer` component, as well as `static contextType = ThemeContext`. Which is not possible with the current implementation.

Author: michalchudziak

src/createTheming.js

@@ -41,6 +41,7 @@ export default function createTheming<T: Object>(
   };
 
   return {
+    ThemeContext,
     ThemeProvider,
     withTheme,
     useTheme,