`env` variable is not passed as first parameter to configuration function

Explain the problem

parallel-webpack is not passing an env parameter when configuration uses a function of the form:

module.exports = (env) => {
    console.log("env: ", env);  // Prints: env: {}
    return myConfig;  
} 

Expected Behaviour

A valid env should be passed as first parameter to the configuration function.

Actual Behaviour

env is an empty dictionary.

Steps to reproduce

1. Install parallel-webpack locally (using yarn, in my case)
2. Create a simple module with a configuration function of the form given above
3. Invoke as shown below:

$ ./node_modules/.bin/parallel-webpack --env.production=true

Provide your Environment details

• Node version: v8.12.0
• Operating System: Ubuntu 14.04.1 LTS
• webpack version: 4.16.5
• parallel-webpack version: 2.3.0