Media objects in newsfeed

Some blogs are art-centric and have posts with no text at all, sometimes even without titles, so it's hard to get them into newsfeed. Also all popular sites such as facebook and twitter have media objects in newsfeed, and it's pretty.
I propose two new fields in the [`feedFollow`](https://zeronet.readthedocs.io/en/latest/site_development/zeroframe_api_reference/#plugin-newsfeed) command:

- `media_url`: url of the media object
- `media_type`: one of `{'image', 'video', 'audio'}`

I think one media object per record would be enough. If user is interested, he can follow the link to view all content.
Please, share your thoughts.

Start with the feedFollow command in the linked ZeroFrame API reference and trace its implementation and existing tests. Determine where feed records are assembled and how media URLs and types would be represented; the work is done when the API behavior and coverage clearly support the proposed media fields.

索引モデルが issue の本文から書いたものです。