Add headers support

[StevenLangbroek]

I'm switching from using a GraphQL based service to my own CMS installation, and its security requirements are different, so I needed custom headers :). Some notes on this PR:

• I'm assuming Object.assign, array destructuring and arrow function support, so Node 6+. Since current LTS is 6, and it'll soon be Node 8, this seems like a reasonable assumption to me. If you disagree, please let me know and I'll refactor to ES5.
• The output of --help for README.md is differently sorted than yours... No idea why, wasn't sure whether you'd like to maintain the old order...
• The changelog appears to be out of date, so I didn't bump versions anywhere.

Furthermore, I think you're reaching the point where it might be good to add some integration-level testing (mocking out the network / fs), and some convention etc... Would you like me to add eslint, editorconfig, prettier and basic set up for more extensive testing?

[StevenLangbroek]

Hmm... the more I think about this, the more I lean towards just supporting the graphql-config standard. There's a parser available too. Thoughts?

Edit: I'd gladly do the legwork for this.

[samhagan]

This is a feature that we would like also. Either directly via headers flag or via .graphqlrc file. A work around at this point is to run the introspection query yourself, write to file, then pass the json to gql2flow.

[StevenLangbroek]

@samhagan if you want i have some time over the weekend, i could whip up a PR for using graphql-config...

[samhagan]

@StevenLangbroek one thing to consider, not related to the headers, is other configuration this package may need. For example, we're using custom scalar types that by default result in any flow types -- these can be fixed by passing many -t flags, but would be better configured in a .gql2flowrc file that could capture any configuration needs.