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.