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/org_test.go

@@ -14,7 +14,7 @@ func TestOrgWriter(t *testing.T) {
 	for _, path := range orgTestFiles() {
 		expected := fileString(path)
 		reader, writer := strings.NewReader(expected), NewOrgWriter()
-		actual, err := NewDocument().Parse(reader).Write(writer)
+		actual, err := NewDocument().SetPath(path).Parse(reader).Write(writer)
 		if err != nil {
 			t.Errorf("%s\n got error: %s", path, err)
 			continue