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/drawer.go

@@ -89,3 +89,6 @@ func (d *PropertyDrawer) Get(key string) (string, bool) {
 	}
 	return "", false
 }
+
+func (n Drawer) String() string         { return orgWriter.nodesAsString(n) }
+func (n PropertyDrawer) String() string { return orgWriter.nodesAsString(n) }