Descriptions as strings in GraphQL SDL

[cjoudrey]

Fixes #77

As per graphql/graphql-js#927, descriptions are now strings instead of comments in the SDL.

For example:

Before

# Query
type Query {
  # A
  a: String
}

Now

"Query"
type Query {
  "A"
  a: String
}

Moving forward schemas will need to use this syntax. In order to provide a graceful upgrade path, users can use the --comment-descriptions CLI option to use the old way of defining descriptions for types.