[PATCH] feat: support more clippers

[Greg Minshall]

hi.

for this, and your exa patch, *i* (who have nothing to do with this
project, other than being a happy user) would prefer to have command
line switches, or some sort of PASS_XCLIP_COMMAND / PASS_LS_COMMAND
environmental variables.

i can imagine scenarios where something named, e.g., "lemonade", does
something other than the desired thing (and, in so doing, might disclose
sensitive information).

cheers, Greg