Add support for extending writers

Go does not support inheritance, just composition. While composition with type
embedding (i.e. forwarding method calls to the embedded type) can replace
inheritance for most use cases this is not one of them. We really want to
overwrite methods so that method calls from inside the base writer also use the
custom methods ouf our extending writer - naive embedding does not work here
as the this in this.WriteText refers to the embedded type rather than the outer
extending type (see open recursion).

A simple solution is to make a reference of the extending type
available from the extended type and use that for nested method calls. We'll go
with that one as it does not require huge code changes. Another solution would
be to flatten the writing process and not use nested method calls - this is
what blackfriday does. Assuming the current solution works I feel it's cleaner
and keeps the ugliness of simulating inheritance with composition contained to
a small portion of the code while blackfridays approach requires all write
methods to be written in a flat style (i.e. not do nested calls to write by
being called twice with entering / leaving). The current solution becomes ugly
if we want to do multiple levels of extending but i don't expect that to be a
valid use case - if it turns out to be one we can always adapt to it
later. YAGNI.

org/html_writer.go

@@ -14,12 +14,14 @@ import (
 
 // HTMLWriter exports an org document into a html document.
 type HTMLWriter struct {
-	strings.Builder
-	document           *Document
+	ExtendingWriter    Writer
 	HighlightCodeBlock func(source, lang string) string
-	htmlEscape         bool
-	log                *log.Logger
-	footnotes          *footnotes
+
+	strings.Builder
+	document   *Document
+	htmlEscape bool
+	log        *log.Logger
+	footnotes  *footnotes
 }
 
 type footnotes struct {
@@ -79,6 +81,13 @@ func (w *HTMLWriter) nodesAsString(nodes ...Node) string {
 	return tmp.String()
 }
 
+func (w *HTMLWriter) WriterWithExtensions() Writer {
+	if w.ExtendingWriter != nil {
+		return w.ExtendingWriter
+	}
+	return w
+}
+
 func (w *HTMLWriter) Before(d *Document) {
 	w.document = d
 	w.log = d.Log