feat(accordion): add templateUrl support

[wesleycho]

• Add ability to override the template of the accordion and accordion-group directives on an instance by instance basis

[Foxandxss]

I am not any sure we want this kind of upgrades. The whole point of having the tpls is to include the templates. You can always use the other version if you don't want the templates.

Still, you can override the templates in the tpls version with $templateCache easily.

[wesleycho]

This would let you override it on a case-by-case basis - say you want one template for one instance and another for a different one

[Foxandxss]

Uhmm, uhmmm. I can see that.

I would put in the doc that it only accepts a string, no interpolation.

[wesleycho]

Alright, I'll make the change - my test has a bit of a hack in it btw. Is it fine or do you have a better recommendation there?