Refactor Writer Interface

The existing approach made it hard to extend existing writers.
With this change, replacing individual methods of a writer is possible by
embedding it.

Sharing the WriteNodes function also removes some unnecesseray duplication, so
win win.

org/writer.go

@@ -0,0 +1,91 @@
+package org
+
+import "fmt"
+
+// Writer is the interface that is used to export a parsed document into a new format. See Document.Write().
+type Writer interface {
+	Before(*Document) // Before is called before any nodes are passed to the writer.
+	After(*Document)  // After is called after all nodes have been passed to the writer.
+	String() string   // String is called at the very end to retrieve the final output.
+
+	WriteKeyword(Keyword)
+	WriteInclude(Include)
+	WriteComment(Comment)
+	WriteNodeWithMeta(NodeWithMeta)
+	WriteHeadline(Headline)
+	WriteBlock(Block)
+	WriteExample(Example)
+	WriteDrawer(Drawer)
+	WritePropertyDrawer(PropertyDrawer)
+	WriteList(List)
+	WriteListItem(ListItem)
+	WriteDescriptiveListItem(DescriptiveListItem)
+	WriteTable(Table)
+	WriteHorizontalRule(HorizontalRule)
+	WriteParagraph(Paragraph)
+	WriteText(Text)
+	WriteEmphasis(Emphasis)
+	WriteStatisticToken(StatisticToken)
+	WriteExplicitLineBreak(ExplicitLineBreak)
+	WriteLineBreak(LineBreak)
+	WriteRegularLink(RegularLink)
+	WriteFootnoteLink(FootnoteLink)
+	WriteFootnoteDefinition(FootnoteDefinition)
+}
+
+func WriteNodes(w Writer, nodes ...Node) {
+	for _, n := range nodes {
+		switch n := n.(type) {
+		case Keyword:
+			w.WriteKeyword(n)
+		case Include:
+			w.WriteInclude(n)
+		case Comment:
+			w.WriteComment(n)
+		case NodeWithMeta:
+			w.WriteNodeWithMeta(n)
+		case Headline:
+			w.WriteHeadline(n)
+		case Block:
+			w.WriteBlock(n)
+		case Example:
+			w.WriteExample(n)
+		case Drawer:
+			w.WriteDrawer(n)
+		case PropertyDrawer:
+			w.WritePropertyDrawer(n)
+		case List:
+			w.WriteList(n)
+		case ListItem:
+			w.WriteListItem(n)
+		case DescriptiveListItem:
+			w.WriteDescriptiveListItem(n)
+		case Table:
+			w.WriteTable(n)
+		case HorizontalRule:
+			w.WriteHorizontalRule(n)
+		case Paragraph:
+			w.WriteParagraph(n)
+		case Text:
+			w.WriteText(n)
+		case Emphasis:
+			w.WriteEmphasis(n)
+		case StatisticToken:
+			w.WriteStatisticToken(n)
+		case ExplicitLineBreak:
+			w.WriteExplicitLineBreak(n)
+		case LineBreak:
+			w.WriteLineBreak(n)
+		case RegularLink:
+			w.WriteRegularLink(n)
+		case FootnoteLink:
+			w.WriteFootnoteLink(n)
+		case FootnoteDefinition:
+			w.WriteFootnoteDefinition(n)
+		default:
+			if n != nil {
+				panic(fmt.Sprintf("bad node %#v", n))
+			}
+		}
+	}
+}