Added goto-offset.

Useful for debugging Codex stuffs which deal in file offsets rather than
rows/cols.
Allow editor geometry to be specified via sysprop.

-Dgeometry=widthxheight+x+y

Width and height are in characters, x and y in pixels.

I don't expect this to be widely used, but it allows me to specify custom
geometry when I'm forking off a test editor, which I do just shy of a billion
times a day.