[Filter] add filter to include local Markdown file via Markdown links in paragraphs

[cagix]

include-mdfiles.lua – filter to include local Markdown files via links

for each link to local Markdown file in start document:
    (1) read file
    (2) "fix" links to local images, i.e. prepend include path
    (3) process links to local Markdown files in paragraphs and include content (recursively)
        - foreach Para:
            - prepare new empty block list (result), add new current block (empty Para)
            - foreach inline in current Para:
                - if not link: append inline to current block's content
                - if link:
                    - read link.target (file), process content and append resulting blocks to block list
                    - add new current block (empty Para) for remaining inlines of current block
            - return block list to replace current Para

warning: this won't handle endless recursion!

fixes #80