metainfo-to-news: Change text output format to be Markdown

[pwithnall]

A lot of projects use Markdown formatting for their NEWS file now, since it’s human-readable, but also well defined as machine readable, and is rendered automatically when viewed online in GitHub or GitLab.

It would be useful if appstreamcli metainfo-to-news outputted valid Markdown, either by changing what the text format outputs, or adding a new markdown output format.

The main issue I’ve noticed with the existing text format is that the underline for the header is ~~~~~~ rather than the ====== or ------ stipulated by Markdown. I’m not sure if any other changes in the formatting would be required to make it valid Markdown.

[ximion]

[Markdown is] well defined as machine readable

That's quite the statement given the huge amount of different styles and interpretations, depending on which Markdown flavor you have ^^

But having this would indeed make sense, possibly even as default. appstreamcli metainfo-to-news already has a --format flag anyway, so adding a Markdown option should be possible.
The only thing I do remember that would need to be changed is indeed the tilde to some other header separator, but while we're at it, adding ### to sections to render them properly may also be useful.
The only question is whether news-to-metainfo should also be able to read the new Markdown format. I think the answer to that question is yes, but it will make this task a bit more annoying to implement ;-)

[pwithnall]

Thank you!