Added extra format specs for arrowheads, especially how they join up with `+`.
Added extra format specs for arrowheads, especially how they join up with `+`.

Note that this is an instance where the formatting of a character
(e.g. `>`) can go *outside* of its grid cell when it is hooked into a
compatible neighbor. (Think of it like how ligatures are done in
certain fixed width fonts like hasklig.)

pnkfelix/mon-artiste