Add support for --graph flag

[zionts]

This PR adds support for the --graph (alias -g) flag to all commands. This will allow users to specify a graph without needing to create an apollo.config.js file, which will lead to a smoother on-boarding ramp for first-time users. Additionally, this PR contains a bug fix for apollo.config.js files of the service type, where specifying a variant is supported equally as to client projects.

For example, a config like so:

module.exports = {
  service: {
    name: github@prod
  }
}

will be parsed as graph = github , variant = prod, just as with client projects (which were supported in that fashion before this PR)

There aren't tests for any of the client: commands, but I added a test for the additional functionality and did manual QA of my changes. Here is a small sampling and happy to provide additional:

service:list working with a --graph flag and a user token:

client:check working with a --graph flag and an apollo.config.js that does not have a service key:

client:codegen working with a --graph flag and an apollo.config.js that does not have a service key:

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.

[JakeDawkins]

Love it!! 🎉