add Extensible class to enyo/utils
add Extensible class to enyo/utils

Allows functions to be extensible in the same way as kind.inherit.
Primarily, this allows us to augment or override core functionality
elsewhere in the framework.

Issue: ENYO-1840
Enyo-DCO-1.1-Signed-off-by: Ryan Duffy (ryan.duffy@lge.com)

PKRoma/enyo,PKRoma/enyo,enyojs/enyo,enyojs/enyo