AMP List element skeleton.

[dvoytenko]

Partial for #657.

This PR essentially brings amp-list and amp-mustache elements to a functional state. The "mustache" experiment has to be on to try it.

Some of the key outstanding questions here:

1. Data attributes vs plain attributes? "data-src" and "data-credentials" are used in this component, but I'm wondering if it'd be better to drop "data-" here.
2. We will need to refactor URL substitutions that feeds into amp-pixel to use it here as well
3. This implementation is missing a fallback for when the AMP runtime refuses to change the height
4. Some of the height change rules might need to be expanded to cover "at the footer" case where the element is within the viewport but very close to the footer.

/cc @erwinmombay

[dvoytenko]

Also added URL substitutions.

[cramforce]

So, do we really need to special case lists?

[dvoytenko]

You mean, do we need an amp-list vs a generic amp-render? If so, here are the reasons why I thought this was useful:

1. It's actually more direct with the original request
2. Clear how to extend it to an infinite scroll case (someone mentioned that as well)
3. Semantics of changing height are more clear
4. I expect as follow up we can add similar strategy to amp-carousel and such

[dvoytenko]

PTAL

[cramforce]

Add something

[dvoytenko]

Done

[cramforce]

LGTM