Add pandoc-include (https://github.com/DCsunset/pandoc-include)

[cjkoenig]

Add support for splitted documents and code files

[daamien]

@tarleb I'm approving this with 2 minor comments:

• We already include 2 Lua filters named include-code-files and include-files that serve the same purpose. However the DCSunset filter seems much more popular, so I agree it's useful but redundant addition.

• I would be nice to add a basic test file for this filter. we can add it later though

[tarleb]

Thanks!

[cjkoenig]

Thank you for adding this! (I was not aware about the existing lua-filters)