Specifying `position:left` could display an Anno off-screen

[iamdanfox]

... if the target is already on the left of the viewport, (e.g. on iPhones).

Hat tip to WiseWeasel

Users shouldn't specify these off-screen positions, but perhaps we should put a warning/error on the console?

[ndtreviv]

I know it's another dependency, but certain other tour libraries use tether (http://tether.io/) to make sure that the annotation stays with the target element. Tether has an option that will flip the annotation to another attachment type if it's showing off-screen.