Before this commit, creating a sub component with
t-att-style/t-att-class attributes or with t-on- event handlers would
override the *hook* object rendered by the sub component.
This is an issue for some directives, such as t-ref, which defines
hook functions.
This commit fixes the issue by checking for an override, and wrapping
the hooks in a function that calls each defined hook.
closes#638
Géry Debongnie