vg: provide a vg(la)tex backend

as evocated in #228, it would be great to support all the LaTeX (sub|super)scripts and formulae.

perhaps go the extra mile and provide a vg/vgtex backend?

• generate the .tex file
• piggy back on pgf/TikZ or pgfplots

https://www.sharelatex.com/learn/Pgfplots_package http://mirror.ibcp.fr/pub/CTAN/graphics/pgf/contrib/pgfplots/doc/pgfplots.pdf http://mirror.ibcp.fr/pub/CTAN/graphics/pgf/base/doc/pgfmanual.pdf