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/block.go

@@ -79,3 +79,6 @@ func trimIndentUpTo(max int) func(string) string {
 		return line[i:]
 	}
 }
+
+func (n Example) String() string { return orgWriter.nodesAsString(n) }
+func (n Block) String() string   { return orgWriter.nodesAsString(n) }

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() {

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) }

org/footnote.go

@@ -54,3 +54,5 @@ func (fs *Footnotes) Ordered() []FootnoteDefinition {
 	}
 	return append(definitions, inlineDefinitions...)
 }
+
+func (n FootnoteDefinition) String() string { return orgWriter.nodesAsString(n) }

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) }

org/inline.go

@@ -274,3 +274,11 @@ func (l RegularLink) Kind() string {
 	}
 	return "regular"
 }
+
+func (n Text) String() string              { return orgWriter.nodesAsString(n) }
+func (n LineBreak) String() string         { return orgWriter.nodesAsString(n) }
+func (n ExplicitLineBreak) String() string { return orgWriter.nodesAsString(n) }
+func (n StatisticToken) String() string    { return orgWriter.nodesAsString(n) }
+func (n Emphasis) String() string          { return orgWriter.nodesAsString(n) }
+func (n FootnoteLink) String() string      { return orgWriter.nodesAsString(n) }
+func (n RegularLink) String() string       { return orgWriter.nodesAsString(n) }

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) }

org/list.go

@@ -108,3 +108,7 @@ func (d *Document) parseListItem(l List, i int, parentStop stopFn) (int, Node) {
 	}
 	return i - start, ListItem{bullet, status, nodes}
 }
+
+func (n List) String() string                { return orgWriter.nodesAsString(n) }
+func (n ListItem) String() string            { return orgWriter.nodesAsString(n) }
+func (n DescriptiveListItem) String() string { return orgWriter.nodesAsString(n) }

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) }

org/table.go

@@ -126,3 +126,5 @@ func isSpecialRow(rawColumns []string) bool {
 	}
 	return isAlignRow
 }
+
+func (n Table) String() string { return orgWriter.nodesAsString(n) }