Added `columns`.

When rendering some non-classically pretty printed items (such as the clang-style diagnostics used by trifecta) it is helpful to know the width the document is being formatted to. I had this functionality in `wl-pprint-terminfo`, and its fairly trivial to add to `ansi-wl-pprint`.

I've attached a patch that adds this functionality to the 'undocumented' function set.