Support for front matter?
Is there going to be support for files with front matter, especially Markdown? These are a block of YAML wrapped in triple-hyphens (---), immediately preceding the content itself, that contains metadata. This is useful for Markdown files meant to be processed by applications such as pandoc, Hugo, or Jekyll, to name a few.
While it would be nice for Editorial to do syntax highlighting for front matter, just having it be aware so it doesn't try to format it as Markdown would be good in itself. An example of the usefulness is in the case of YAML comments, which start with # — Editorial reads this and treats it as a level-1 Markdown header.
Some applications also use TOML wrapped in triple-pluses (+++), though this seems to be less common. One application that uses this is Hugo.