Support z+ and z^

[openjck]

From the Vim documentation:

                            *z+*
z+          Without [count]: Redraw with the line just below the
            window at the top of the window.  Put the cursor in
            that line, at the first non-blank in the line.
            With [count]: just like "z<CR>".

                            *z^*
z^          Without [count]: Redraw with the line just above the
            window at the bottom of the window.  Put the cursor in
            that line, at the first non-blank in the line.
            With [count]: First scroll the text to put the [count]
            line at the bottom of the window, then redraw with the
            line which is now at the top of the window at the
            bottom of the window.  Put the cursor in that line, at
            the first non-blank in the line.