Add basic support for statistic tokens (e.g. [100%] [1/1])

Org mode does not care where those tokens are when it comes to the
export (afaict). We'll do the same.

(They should only be in the first line of a list item or a headline)

org/org.go

@@ -86,6 +86,8 @@ func (w *OrgWriter) writeNodes(ns ...Node) {
 			w.writeText(n)
 		case Emphasis:
 			w.writeEmphasis(n)
+		case StatisticToken:
+			w.writeStatisticToken(n)
 		case LineBreak:
 			w.writeLineBreak(n)
 		case ExplicitLineBreak:
@@ -285,6 +287,10 @@ func (w *OrgWriter) writeEmphasis(e Emphasis) {
 	w.WriteString(borders[1])
 }
 
+func (w *OrgWriter) writeStatisticToken(s StatisticToken) {
+	w.WriteString(fmt.Sprintf("[%s]", s.Content))
+}
+
 func (w *OrgWriter) writeLineBreak(l LineBreak) {
 	w.WriteString(strings.Repeat("\n"+w.indent, l.Count))
 }