With GitHub’s new GraphQL API, you can send a single request to list of files with their content. Try it out on https://developer.github.com/v4/explorer/

list_offlinefirst_posts.graphql

{
  organization(login: "offlinefirst") {
    repository(name: "offlinefirst.org") {
      object(expression: "gh-pages:_posts/") {
        ... on Tree {
          entries {
            object {
              ... on Blob {
                text
              }
            }
          }
        }
      }
    }
  }
}