What steps will reproduce the problem?
1. emmiter << YAML::Null

What is the expected output? What do you see instead?

{{{
~
}}}

What version of the product are you using? On what operating system?
0.2.7

Please provide any additional information below.

It'd be nice if you could modify how YAML::Null is represented, I have a file I 
have to generate that way:

{{{
- foo: 123
- bar:
- baz: "yeah"
}}}

To mean, "no values" for "bar"... right now this emits "- bar: ~".