Add NullValue to allow sending null in a value literal

[clery]

I noticed null was not a valid value to return anywhere in a ParseLiteral, so I took a look at the way the parsing was done with graphql-js and tried reimplementing their NullValue.

There may be other stuff to do, for example fixing #457 to allow sending null to any scalar type as an argument.

Feel free to tell me if anything looks bad or whatever 🙂

[coveralls]

Coverage remained the same at 92.17% when pulling 1967934 on clery:master into c13c47a on graphql-go:master.

[clery]

Just noticed that there already is a pull request about this #401