Visit self to print the AST content

Property definitions

markdown2 $ MdASTPrinter :: print_ast
	# Visit `self` to print the AST content
	fun print_ast(node: MdNode) do
		print "{"  " * indent}{node}"
		indent += 1
		node.visit_all(self)
		indent -= 1
	end
lib/markdown2/markdown_ast.nit:171,2--177,4