Add CdoConfirm and LdoConfirm options

This patch adds `:CdoConfirm` and `:LdoConfirm`, which do the same thing as `:Cdo` and `:Ldo`, except that they ask for confirmation before changing each file.

When I'm running the same macro over a long list of files (say, changing a bunch of specs to use newer syntax), there are inevitably some that were formatted differently initially and are messed up by the macro. Having the confirmation option makes it much easier to do bulk edits safely.