Support reStructuredText doctest blocks
LabeledFlowabled: add option to allow the label to spillover

When the 'wrap_label' LabeledFlowable style options is set to False, a
label that is wider than 'label_max_width' will spill over into the
content area, pushing the content down.
Make VirtualContainer take up the available width by default
ExpandingContainer: default max_height to the parent's remaining_height

VirtualContainer still has infinite max_height.
DownExpandingContainer: default top to the parent's cursor position