Allow hooking into the template instantiation process

[jonrimmer]

At present, Polymer looks for the first template defined in an element and instantiates it automatically within a shadow root. It would be useful to be able to override this behaviour if, for example, I wanted to include several templates and choose which of them to use based on particular conditions, such as the attributes values.

[sjmiles]

Thanks Jon.

[blittle]

👍

[sjmiles]

This is now supported via overriding fetchTemplate method on a polymer-element:

    fetchTemplate: function(elementElement) {
      return elementElement.querySelector('template'); 
    }

There are a few other methods you can override to customize set up behavior, like parseElements. Usage is likely non-obvious, but remains To Be Documented. Here is a link to the relevant code in the meantime: https://github.com/Polymer/polymer/blob/stable/src/instance/base.js#L39

[sorvell]

Since the functionality is there but remains to be documented, closing in favor of #305.