Support GraphQL variables

[bookmoons]

Adds support for GraphQL variables in a GraphQL body translation.

Adds new test cases for GraphQL bodies.

There's also a live example that can be run against the GitHub API. It pulls a few repo names:

• node bin/postman-to-k6.s example/v2/github-graphql.json >test.js
• In GitHub developer settings grab a personal access token, one with repo access. Paste it as the value of GITHUB_TOKEN at the top.
• k6 run test.js

It should print an info line with an object like this:

{"data":{"viewer":{"name":null,"repositories":{"nodes":[{"name":"ulid"},{"name":"gojay"},{"name":"yaegi"}]}}}}

Closes #38.

[robingustafsson]

Have tested this and works great. Merging. Thanks @bookmoons!

[bookmoons]

Thanks!