Optionally include GraphQL Query / Mutation in querygen output

[bengsparks]

$ cynic querygen --query query.graphql  --schema schema.graphql

Currently "only" outputs Rust structs with the correct #[derive(...)]s attached.
By prepending the contents of query.graphql to the output within a markdown code block, it becomes possible to document the generated code automatically.

Example:

schema.graphql:

type Query {
  helloWorld: String!
}

query.graphql

query HelloWorld {
    helloWorld
}

cynic querygen --query query.graphql --schema schema.graphql

#[derive(cynic::QueryFragment, Debug)]
#[cynic(graphql_type = "Query")]
pub struct HelloWorld {
    pub hello_world: String,
}

cynic querygen --query query.graphql --schema schema.graphql --structdoc

/// ```gql 
/// query HelloWorld {
///     helloWorld
/// }
/// ```
#[derive(cynic::QueryFragment, Debug)]
#[cynic(graphql_type = "Query")]
pub struct HelloWorld {
    pub hello_world: String,
}

cynic querygen --query query.graphql --schema schema.graphql --moddoc

//! ```gql 
//! query HelloWorld {
//!     helloWorld
//! }
//! ```


#[derive(cynic::QueryFragment, Debug)]
#[cynic(graphql_type = "Query")]
pub struct HelloWorld {
    pub hello_world: String,
}