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/paragraph.go

@@ -41,3 +41,6 @@ func (d *Document) parseParagraph(i int, parentStop stopFn) (int, Node) {
 func (d *Document) parseHorizontalRule(i int, parentStop stopFn) (int, Node) {
 	return 1, HorizontalRule{}
 }
+
+func (n Paragraph) String() string      { return orgWriter.nodesAsString(n) }
+func (n HorizontalRule) String() string { return orgWriter.nodesAsString(n) }