Add lualatex support

Lualatex should be handled nearly as pdflatex as reported
by Andreas Beckmann in https://bugs.debian.org/815836

A new LUALATEX flavor is now defined.
To use it instead of pdflatex, one just have to set 'LU_FLAVORS=LUALATEX'
Of course, flavors can be chosen per document if need be (as in the example directory)