add ability to turn off the default behavior of allowing bindings to propagate undefined

Enyo-DCO-1.1-Signed-Off-By: Cole Davis (cole.davis@lge.com)
Merge pull request #567 from enyojs/binding-update

add ability to turn off the default behavior of allowing bindings to pro...

Reviewed-By: Ben Combee (ben.combee@lge.com)