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.

org/html.go

@@ -11,7 +11,6 @@ import (
 
 type HTMLWriter struct {
 	stringBuilder
-	document           *Document
 	HighlightCodeBlock func(source, lang string) string
 }
 
@@ -65,6 +64,8 @@ func (w *HTMLWriter) writeNodes(ns ...Node) {
 		switch n := n.(type) {
 		case Keyword:
 			w.writeKeyword(n)
+		case Include:
+			w.writeInclude(n)
 		case Comment:
 			continue
 		case NodeWithMeta:
@@ -144,6 +145,10 @@ func (w *HTMLWriter) writeKeyword(k Keyword) {
 	}
 }
 
+func (w *HTMLWriter) writeInclude(i Include) {
+	w.writeNodes(i.Resolve())
+}
+
 func (w *HTMLWriter) writeFootnoteDefinition(f FootnoteDefinition) {
 	n := f.Name
 	w.WriteString(`<div class="footnote-definition">` + "\n")