Introduce receptacles

Introduce receptacles

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.