[MER-2418] Make it easy to detect a PR markdown

[draftcode]

This is a nice to have and a harmless change. The temp file created for
the pull-request title and body now has an extension '.av.md'. This is
anyway recognized as a markdown file for most of the editors, but this
extra 'av' extension allows us to have a special file type that supports
'%%' as a comment.

As an example, added a Vim plugin that recognize this file type and
treats '%%' as a comment.

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[draftcode]

This makes Vim render the PR body like this:

[draftcode]

I'm fine with excluding the Vim plugin part. I can keep it local. But the PR temp file change is anyway harmless.

[jainankit]

This is great. We should also document this - suggest that folks can use the vim plugin or build their own for their IDE (i future we can do a VS code plugin for this).

[twavv]

I wonder if it's possible to include these as extra files installed by (e.g.,) the brew cask.

[draftcode]

I wonder if it's possible to include these as extra files installed by (e.g.,) the brew cask.

It's probably a bit difficult. But since Vim has relatively established plugin install system now, I think it's easy to distribute this as a Vim plugin (by moving these files to another repo).