Support basic #+INCLUDE (src/example/export block only)

including org files is more complex - e.g. footnotes need to be namespaced to
their source file. org does this by prefixing each included files footnotes
with a number - but even that is not enough as it doesn't guarantee
uniqueness.

As I don't have a usecase for it, I'll avoid the additional complexity for
now.

main.go

@@ -21,20 +21,22 @@ func main() {
 		log.Println("USAGE: org FILE OUTPUT_FORMAT")
 		log.Fatal("supported output formats: org, html, html-chroma")
 	}
-	bs, err := ioutil.ReadFile(os.Args[1])
+	path := os.Args[1]
+	bs, err := ioutil.ReadFile(path)
 	if err != nil {
 		log.Fatal(err)
 	}
-	r, out, err := bytes.NewReader(bs), "", nil
+	out, err := "", nil
+	d := org.NewDocument().SetPath(path).Parse(bytes.NewReader(bs))
 	switch strings.ToLower(os.Args[2]) {
 	case "org":
-		out, err = org.NewDocument().Parse(r).Write(org.NewOrgWriter())
+		out, err = d.Write(org.NewOrgWriter())
 	case "html":
-		out, err = org.NewDocument().Parse(r).Write(org.NewHTMLWriter())
+		out, err = d.Write(org.NewHTMLWriter())
 	case "html-chroma":
 		writer := org.NewHTMLWriter()
 		writer.HighlightCodeBlock = highlightCodeBlock
-		out, err = org.NewDocument().Parse(r).Write(writer)
+		out, err = d.Write(writer)
 	default:
 		log.Fatal("Unsupported output format")
 	}