Show keep screen on in reader settings. Closes #146

app/src/main/res/xml/pref_reader.xml

@@ -17,6 +17,10 @@
         android:key="@string/pref_custom_brightness_key"
         android:defaultValue="false" />
 
+    <SwitchPreference android:title="@string/pref_keep_screen_on"
+        android:key="@string/pref_keep_screen_on_key"
+        android:defaultValue="true" />
+
     <SwitchPreference android:title="@string/pref_seamless_mode"
         android:key="@string/pref_seamless_mode_key"
         android:defaultValue="true" />