Include files filter updates

[lmunch]

This patchset keeps the paths to images relative and also improves the usage in combination with the include-code-files.lua filter

[lmunch]

@tarleb Please review

[tarleb]

Thanks, looks good! Can we add a test for this?

[lmunch]

Sure, I can add a test. I will look at it later this week.

[lmunch]

@tarleb I have updated the test and documentation, but the CI fails in math2svg which seems to be unreleated.

BTW, awesome work with pandoc, pandoc/latex docker image etc. Use it everyday, highly appreciated.

[tarleb]

Awesome, thanks! I'll look into the math2svg problems.

[tarleb]

It seems that the math2svg problems are caused by changes in the npm ecosystem. It's probably enough to configure a Ubuntu PPA to get a newer node version installed in our testing container. I'll try if that fixes it, and just merge regardless if it doesn't.

[gabyx]

@lmunch : The relative path changes is generally a good idea, but its really bit too intrusive because

• generally one never uses absolute paths in markdown documents, so one cannot have include paths relative to the working directory anymore which is sad (since images might be inside files/img.png which is relative to the current working dir.

This feature needs probably more thoughts:

A variant with absolute paths like:

```{.include}
${env:ROOT_DIR}/chapters/A.md

will work if `${env:ROOT_DIR}` is replaced beforehand with the environment variable `ROOT_DIR` which points to the root of the document tree.

In this way one can still write multimarkdown documents.

Or maybe (`-M "includes-relative-to-cwd=true")

````md
```{.include .relative-to-current}
subdir/A.md

{.relative-to-current}

[gabyx]

I added this feature in #177. See the Readme under transclusion.