Support block string literal

[Totktonada]

graphql/graphql-spec#327

It would be especially useful for long expressions we'll support in the scope of #13.

[Totktonada]

I'm going to archive the repository. I'll proceed as follows:

• Mark all open pull requests with the archived label and close.
• Mark all open issues with archived and wontfix labels and close.
• Archive the repository.

Consider the following projects:

• https://github.com/tarantool/graphql
• https://github.com/tarantool/graphqlapi
• https://github.com/tarantool/graphqlide