Add virtcol parameter for col_current segment

The virtual column number is also default. I doubt anybody here ever
wanted to see byte offset. When it comes to a questions like alignment,
forced text width and so on only virtual columns matter. It would be
good to optionally take concealed characters into account, but vim does
not provide a way to do so (well, except recent screencol() added solely
for testing which would be hard, but not impossible, to adapt to such
needs).

Closes #25.