Update document content or frontmatter.

Two modes, selected by which flags are present:

  Body-overwrite mode (-k KEY -c CONTENT)
    Rewrites the full markdown body of one document. Use '-' as --content
    to read from stdin.

  Frontmatter-mutation mode (--filter EXPR | -k KEY) + --set / --unset
    Applies $set / $unset operators to every document matched by the
    filter. -k KEY is sugar for
    '$key: K'; combine with --filter to AND further constraints.

Body and mutation flags cannot be combined; pass exactly one mode.
