Dev vg x11

- add an X11 vg back-end
- imported from github.com/Arrow/vgximg

Also, as you already mentioned, adding this will break go get github.com/gonum/plot/... on systems without X11.
We should add the appropriate build tags and verify that the package is still go gettable on OSX and Windows (actually, I've never tried Plotinum on Windows, anyone know if that works?)