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/document.go

@@ -111,6 +111,11 @@ func (d *Document) Parse(input io.Reader) *Document {
 	return d
 }
 
+func (d *Document) SetPath(path string) *Document {
+	d.Path = path
+	return d
+}
+
 func (d *Document) FrontMatter(input io.Reader, f func(string, string) interface{}) (_ map[string]interface{}, err error) {
 	defer func() {
 		d.tokens = nil