Hello,

This patch adds commands for commenting rectangles and modifies 
`comment-dwim` to use them. This is useful, for example, for commenting 
out the middles of consecutive lines, for which `comment-region` 
wouldn't work. It does this by calling `comment-region` or 
`uncomment-region` on each segment within the rectangle. With this patch,

	word1 word2 word3
	word4 word5 word6

would become

	word1 /* word2 */ word3
	word4 /* word5 */ word6

instead of

	word1 /* word2 word3
	word4 word5 */ word6

as produced by `comment-region` when using `rectangle-mark-mode`.

Thank you.