Avoid scrolling on RFB object focus

Chrome scrolls the view to show as much as possible of the canvas when
we call focus(), which is likely not the desired behaviour.

This also exposes the ability to pass on future options when focusing
the RFB object manually.

docs/API.md

@@ -328,7 +328,14 @@ Keyboard events will be sent to the remote server after this point.
 
 ##### Syntax
 
-    RFB.focus( );
+    RFB.focus( [options] );
+
+###### Parameters
+
+**`options`** *Optional*
+  - A `object` providing options to control how the focus will be
+    performed. Please see [`HTMLElement.focus()`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/focus)
+    for available options.
 
 #### RFB.blur()