Add support for gherkin contexts

#### What are you trying to achieve?

I would like to organize files with steps by context, similar to [codeception](https://codeception.com/docs/07-BDD#Configuration):
```js
"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.