Investigate using the prefix arg for including and excluding the delimiters

[Thomas Kappler]

From a comment on https://jugglingbits.wordpress.com/2010/05/16/announcing-delim-kill-el/: it might be nice to be able to choose whether to kill the delimiters themselves or not. The prefix arg is already used for deciding whether to kill or to copy, but for copying one could always press C-y.