first stab at label box pointers. <option>top</option> on a <label> makes

first stab at label box pointers. <option>top</option> on a <label> makes
such a box:

    _____/\_____
    |   Booo   |
    |__________|

likewise with options bottom, left, right. The size can be set via option
<marker-offset> (analogous to <tape> offsets), which describes the distance
from the base to the peak. Default: 8 px