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/headline.go

@@ -97,3 +97,5 @@ func (parent *Section) add(current *Section) {
 		parent.Parent.add(current)
 	}
 }
+
+func (n Headline) String() string { return orgWriter.nodesAsString(n) }