Block request: Petrified oak slabs

[crubleigh]

While not available for many gamers, the petrified oak slab is an attractive alternative to regular oak wood/slabs as a map color for those of us who do have access to it. This is especially the case for those of us who clear and re-use our map canvas since because it is stone it can be instant mined, compared to wood products which cannot.

[jb2170]

This will probably be possible when adding custom blocks is implemented for #37. Idk if there's a solid deadline in implementing that yet lol, but soon :tm: