Add String() method to Node interface

Being able to very easily get the original [1] Org mode content seems like
something that will come up quite often and is very little code.

[1] it's not really the original content, but rather the pretty printed version
of that - as the semantics don't change it shouldn't matter.

org/list.go

@@ -108,3 +108,7 @@ func (d *Document) parseListItem(l List, i int, parentStop stopFn) (int, Node) {
 	}
 	return i - start, ListItem{bullet, status, nodes}
 }
+
+func (n List) String() string                { return orgWriter.nodesAsString(n) }
+func (n ListItem) String() string            { return orgWriter.nodesAsString(n) }
+func (n DescriptiveListItem) String() string { return orgWriter.nodesAsString(n) }