Move command over to new v2 structure

The following changes need to be made to move the command over to the v2 structure:

[ ] Make sure the correct framework is required:
```
composer require wp-cli/wp-cli:^2
```
[ ] Require the testing framework as a dev dependency:
```
composer require --dev wp-cli/wp-cli-tests:^0
```

[ ] Use the .travis.yml file from wp-cli/wp-cli:

wget https://raw.githubusercontent.com/wp-cli/wp-cli/master/.travis.yml

[ ] Add the default script configuration to Composer file:

"scripts": {
    "lint": "run-linter-tests",
    "phpcs": "run-phpcs-tests",
    "phpunit": "run-php-unit-tests",
    "behat": "run-behat-tests",
    "prepare-tests": "install-package-tests",
    "test": [
        "@lint",
        "@phpcs",
        "@phpunit",
        "@behat"
    ]
},

[ ] Remove scaffolded binary files:

git rm bin/install-package-tests.sh
git rm bin/test.sh

[ ] Remove scaffolded Behat setup:

git rm features/bootstrap/*
git rm features/extra/*
git rm features/steps/*

[ ] Remove scaffolded Behat tags util script:
```
git rm utils/behat-tags.php
```
[ ] Add command packages that are needed for Behat tests as --dev dependencies. The following commands are already available, anything else needs to be explicitly required:
- cli *
- config *
- core *
- eval
- eval-file
- help
[ ] Update all dependencies:
```
composer update
```

[ ] Optional - Add PHPCS rule set to enable CS & compatibility sniffing:

wget https://raw.githubusercontent.com/wp-cli/wp-cli/master/phpcs.xml.dist

[ ] Run and adapt tests to make sure they all pass:
```
composer test
```

wp-cli / export-command

Move command over to new v2 structure #43