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/keyword.go

@@ -158,3 +158,8 @@ func (d *Document) loadSetupFile(k Keyword) (int, Node) {
 	}
 	return 1, k
 }
+
+func (n Comment) String() string      { return orgWriter.nodesAsString(n) }
+func (n Keyword) String() string      { return orgWriter.nodesAsString(n) }
+func (n NodeWithMeta) String() string { return orgWriter.nodesAsString(n) }
+func (n Include) String() string      { return orgWriter.nodesAsString(n) }