Abstract content fetching mechanism

[josh]

Defines a DeferredContentElement.prototype.fetch can be overridden in subclasses to define a custom fetch method. The method takes a String url => Promise(String html).

I think the current poll fetch method is too specific to github's legacy polling design and shouldn't be promoted as the core api.

PollDeferredContentPrototype.fetch = pollFetch;

document.registerElement('poll-deferred-content', {
  prototype: PollDeferredContentPrototype,
  extends: 'deferred-content'
});

<deferred-content is="polled-deferred-content" src="branches.html" wait=1000></deferred-content>

[dgraham]

If there's one dependency worth adding, it's Promises. I was going for zero, but it might be worth it.

This feels a little premature, though. I was hoping to replace more of our .js-deferred-content usage with this element to find out if polling was really needed or not.

[josh]

This feels a little premature, though

Its still useful for overriding for other things. If you wanted timeouts, I think this would be a good place to override.

[josh]

We could also consider specing this as an "abortable" promise extension. If the promise defines .abort(), we'll call it when .src changes and the promise is still pending.

[josh]

If there's one dependency worth adding, it's Promises. I was going for zero, but it might be worth it.

We definitely want promises to help implement #4.

[josh]

Tests pass after merging #6.