Add support for gherkin contexts

[hason]

What are you trying to achieve?

I would like to organize files with steps by context, similar to codeception:

"gherkin": {
  "features": "./features/*.feature",
  "contexts": {
    "default": ["./step_definitions/steps.js"],
    "tag": {
      "fastlogin": ["./step_definitions/fastlogin.js"]
    },
    "role": {
      "admin": ["./step_definitions/admin.js"]
    }
  }
}

What do you get instead?

This functionality could probably be simulated using events.