Add YAML GUI / Tooling for Bloggers and Authors

[kmwallio]

Terminology is different for blogs and books (coverimage, featureimage, date, datetime). It would also help to show which fields matter.

It'd be useful to have a context menu to update YAML headers. It could have tabs for Blog / Novel / Etc.

A selector for choosing a cover or feature image and displaying a preview if a correct one is selected would be cool.