Consider allowing extensions to provide their own Loader animation

[mkhatib]

Currently there doesn't seem to be a way for extensions to provide their own loader animation. So extensions like Apster are (mis)using createPlaceholderCallback to provide their own gif animated image as a branded loader.

Maybe we can consider making the -amp-loader related CSS classes not private and allow extensions to override the animation. Or create a createLoaderElementCallback to allow extensions to provide their own.

[dandv]

Any progress on this?

Alternatively, can we document the current node structure and classes for displaying the loader?

<div class="i-amphtml-loading-container i-amphtml-fill-content">
  <div class="i-amphtml-loader amp-active">
    <div class="i-amphtml-loader-dot"></div>
    <div class="i-amphtml-loader-dot"></div>
    <div class="i-amphtml-loader-dot"></div>
  </div>
</div>