Make a new Interactable for each selector+context

The context should only be set when creating the Interactable:
interact('.drag', { context: element });

The context method only returns the context of the Interactable. It
doesn't change it.
Fix delegated DOM events on selector interactables

Listeners are added to the context Node and event removal has been
fixed.
Fix 'ignoreFrom' for element interactables
Fix context and ignoreFrom settings in IE8

Use `isElement(el)` instead of `el instanceof Element||Node` etc.
Add back context dr.js docs
Bump version to 1.0.17
Merge branch 'better-settings'