implement transclusion

[therealbstern]

This implements a WikiMedia feature they call "transclusion", which pulls one wiki page into another.

Right now, it uses [[filename.md]] to pull in another file.

Flaws:

• No security: no defense against infinite loops. This could be added by limiting the depth of transclusion possible
• No security: no defense against directory traversal when running with --fs.
• No file-type checking: if a JPG contains [[...]] it will try to transclude and break the image

If any of these are too much for you to accept, please let me know and I can see if I can make the implementation more robust.

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[AustinWise]

Thanks for your contribution. I'm not too worried about that the flaws you mention, since this is a single user tool. I think we can address them if they become a problem.

I do request two small changes to syntax. First I think we should use {{filename.md}} as the syntax. It appears that a pretty common syntax for transclusion. If we use a common syntax, people's files are more portable between tools.

Examples:

• https://en.wikipedia.org/wiki/Help:Transclusion
• https://tiddlywiki.com/static/Transclusion%2520in%2520WikiText.html
• https://fletcher.github.io/MultiMarkdown-6/syntax/transclusion.html

I do see other syntaxes:

• https://help.obsidian.md/Linking+notes+and+files/Embed+files : This uses a ![[filename.md]]
• https://docs.moodle.org/403/en/MoodleDocs:Transclusion : This uses [[:filename.md]]

Secondly, for now I think the transclusion should only be allowed to appear on a line by itself.

[AustinWise]

Thanks again. I plan to do a release by this weekend at the latest.