A receptacle is an unfocusable empty leaf node.
Receptacles are used for building a tree without creating windows.
Example:
bspc node -i
bspc node @/ -p east -i
bspc node @/2 -p north -i
bspc rule -a Abc:abc -o node=@/1
bspc rule -a Ijk:ijk -o node=@/2/1
bspc rule -a Xyz:xyz -o node=@/2/2
Fixes#259.
