dmenu_bw: Add dmenu front-end for bwpass

`dmenu_bw` works like `passmenu`: it uses `dmenu` to provide a picker,
and then copies the password for the selected item to the clipboard.

dmenu_bw

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+set -e
+
+
+key="$(bwpass | dmenu)"
+[ -n "${key}" ] || exit 0
+password="$(bwpass show "${key}")"
+[ -n "${password}" ] || exit 1
+printf '%s' "${password}" | xclip -quiet -rmlastnl -sel clip 2> /dev/null &
+sleep 12
+kill $!