Store mf2 properties using mf2_ prefix

[dshanske]

I would like to store select properties in meta. Since we now have the 'standard' of using mf2_ to store these, we can store some properties, and I plan on using a loop, and a filter to decide what to store.

For myself, I want syndication so I can explore deduping POSSE and Webmentions, Comments for future nesting of responses and salmentions, etc.

[dshanske]

Working on this in a fork, but need to tweak to match the Micropub storage methodology for consistency.

[dshanske]

Should be addressed by #85