Add an example with items on the edges of the viewport
Add viewport and viewportPadding options

Handle those options when positioning the tip
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