Allow for 'demo' files in the Lua build script

There is one for l3keys2e, for example, and currently this is not on CTAN.
Not 100% clear where this should go: I'm guessing into doc.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4772 de43f980-851b-0410-b2f7-c40aca1f87e0