Fix test for viewport

Because of the viewport adjustment, if the target is offscreen, then the 
tip will be adjusted to be within the viewport. So I use a fixed 
position to place it within the viewable area.
Change name of function to match viewport
:lipstick:
Remove the tip style after test
Add tests for viewport usage
Update title of the example file
Add docs for `viewport` and `viewportPadding` options