Importing externalAssets and htmlAssets from NPRML

[tamw-wnet]

Many stories retrieved from the NPR Story API refer to tweets, youtube vids, or embedded interactives that are not included in the generated WordPress post. The tweets and youtube vids are attached to the end of the original NPRML "story" as externalAssets (generally as oEmbeds), and the interactives seem to be attached as htmlAssets.

Importing those into postmeta for integration into the themes would be useful. I've made a pull request for the externalAsset import -- #69 -- and I'm in the process of doing another pull req for the htmlAssets.

[tamw-wnet]

noting that I've withdrawn pr #69 because I didn't know the 'layout' response was available in the API. I've resubmitted as #74 to use that layout data.