Add support for <M-BS> as delete-backward-word

Although `<C-W>` is perfectly useful already, one of the readline key bindings I use a lot is `<M-BS>` ("delete-backward-word").  However, implementing this is complicated by the fact that it is apparently impossible(?) to get the actual backspace key within vim.  Attempts to, for example,

        exe "set <F34>=\<Esc>\<BS>"

result in `<F34>` being set to `<Esc><80>fb`, which is probably some internal indirect reference to the termcap variable, and, moreover, doesn't work. Inexplicably, as well, one cannot retrieve the value of &t_xx (&t_fb specifically is what we want here) termcap options in vim script.

As far as I know, in the modern era, terminals generally use either `<C-H>` or `<C-?>` for backspace; and sometimes these are transposed with the bindings for `<Del>`. My brutish solution is to bind both `<Esc><C-H>` and `<Esc><C-?>` to the desired function. The consequence is that, for some terminals, `<M-Del>` might be bound to the same as `<M-BS>`, but I don't think `<M-Del>` means anything in readline anyhow.