proposal: text format support

[aca]

This is follow-up of discussion with @srid , @maralorn

Many developers use plain text document and even draw ascii diagram.

• https://github.com/torvalds/linux/blob/master/README
• https://blog.regehr.org/archives/1653

VIM document is also an good example based on simple text format.

• https://vimhelp.org/

Simply replacing extension to markdown will break the view (like new lines).
Currently only way get plain text view in markdown is using code block inside markdown.

It would be great if we can support txt format with linking, metadata.
For metadata just like markdown format, we can use title yaml block.

[aca]

Recently realized that pandoc actually supports plain text file conversion to markdown format.

https://pandoc.org/demos.html#examples
https://pandoc.org/demo/MANUAL.txt

But not sure about the txt format it supports.