Proposal: add a Library for extracting embed dimensions for common platforms

It'd be nice to have a library for generating AMP snippets from

For example, you provide one of these:

* image URL
* tweet URL / id
* facebook post URL / id
* instagram post / id

and you get the correct AMP code to embed these.

See: https://github.com/ampproject/amp.dev/issues/279

Start by reviewing the related amp.dev issue 279 and the amp-toolbox repository structure, since no implementation files or tests are named here. Define the supported URL or ID inputs and the expected AMP snippet output for each platform; the work is done when those requirements and an implementation scope are established.

Written by the indexing model from the issue text.