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.

Makefile

@@ -22,6 +22,9 @@ case=example
 render:
 	go run main.go org/testdata/$(case).org html | html2text
 
+.PHONY: generate
+generate: generate-gh-pages generate-html-fixtures
+
 .PHONY: generate-gh-pages
 generate-gh-pages: build
 	./etc/generate-gh-pages