Add support for Block String in GraphQL files

[jeremywiebe]

The GraphQL spec added support for multiline strings ("Block Strings") in RFC #327. Then in June 2018 the schema declared support for using Block Strings to write descriptions.

It would be useful if Rewrap supported wrapping Block Strings in .graphql files.

For example:

"""
Defines the return type for `MyMutation`. This mutation does some stuff and then returns the name of the operation that was executed.
"""
type MyMutation {
    name: String!
}

would become

"""
Defines the return type for `MyMutation`. This mutation does some stuff and 
then returns the name of the operation that was executed.
"""
type MyMutation {
    name: String!
}