Implements a custom class generator

When `bin/behat --init` is run, the following class is generated:

``` php
<?php

use Behat\Behat\Tester\Exception\PendingException;
use Drupal\DrupalExtension\Context\RawDrupalContext;
use Behat\Behat\Context\SnippetAcceptingContext;
use Behat\Gherkin\Node\PyStringNode;
use Behat\Gherkin\Node\TableNode;

/**
 * Defines application features from the specific context.
 */
class FeatureContext extends RawDrupalContext implements SnippetAcceptingContext {

  /**
   * Initializes context.
   *
   * Every scenario gets its own context instance.
   * You can also pass arbitrary arguments to the
   * context constructor through behat.yml.
   */
  public function __construct() {
  }

}
```