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/document.go

@@ -41,8 +41,10 @@ type Document struct {
 	Error          error
 }
 
-// Node represents a parsed node of the document. It's an empty interface and can be ignored.
-type Node interface{}
+// Node represents a parsed node of the document.
+type Node interface {
+	String() string // String returns the pretty printed Org mode string for the node (see OrgWriter).
+}
 
 type lexFn = func(line string) (t token, ok bool)
 type parseFn = func(*Document, int, stopFn) (int, Node)
@@ -69,6 +71,7 @@ var lexFns = []lexFn{
 }
 
 var nilToken = token{"nil", -1, "", nil}
+var orgWriter = NewOrgWriter()
 
 // New returns a new Configuration with (hopefully) sane defaults.
 func New() *Configuration {
@@ -84,6 +87,9 @@ func New() *Configuration {
 	}
 }
 
+// String returns the pretty printed Org mode string for the given nodes (see OrgWriter).
+func String(nodes []Node) string { return orgWriter.nodesAsString(nodes...) }
+
 // Write is called after with an instance of the Writer interface to export a parsed Document into another format.
 func (d *Document) Write(w Writer) (out string, err error) {
 	defer func() {