Like selection.select, selection.append and selection.insert can now accept a
function which returns a node. This makes it slightly easier to append or insert
elements whose name is computed from data, or to append elements that already
exist (say from an element pool).

There has been much discussion regarding whether the function should return the
name of the element or the element itself. Returning a name is less work for the
caller, but only supports creating new elements; returning a name is also more
consistent with how D3 defines attribute values, but D3 does not allow attribute
names to be specified as functions. So, it seemed better to opt for consistency
with selection.select and selection.selectAll, which accept functions that
return elements, since this is more expressive. Of course, you can still use
select and selectAll to append elements, but using append to do that directly is
more intuitive.

Related #4 #311 #724 #732 #734 #961 #1031 #1271.