#43 show only arcs inside a node

[Rob Lintern]

It would be nice to be able to choose to see arcs when
nesting in the following ways:

1) Show only the arcs that go between the children (or
descendents) of a particular node, A.

2) Show only the arcs that go from the children (or
descendents) of a node, A, to other nodes outside node A.

3) Show only the arcs that go from a particular node, A, to
other nodes and hide arcs going to/from the descendents
of node A.

4) Show only the arcs that go from other nodes to a
particular node, A, and hide arcs going to/from the
descendents of node A