get tgmpa menu slug and parent from it's own config

[vburlak]

Closes #5 .

Need some testing.

Test's:

• [x] Default TGMPA $parent_slug and $menu. Trying to Install & Activate all plugins.
• [x] Custom TGMPA $parent_slug and $menu. Trying to Install & Activate all plugins.
• [x] Default TGMPA $parent_slug and $menu. Trying only Activate all plugins.
• [x] Custom TGMPA $parent_slug and $menu. Trying only Activate all plugins.

My testing TGMPA environment:

array(
    'name'               => __('Visual Composer', 'orbital_station'), // The plugin name.
    'slug'               => 'js_composer',
    'source'             => get_stylesheet_directory() . '/includes/plugins/js_composer.zip',
    'required'           => true,
    'version'            => '4.9.2',
    'force_activation'   => false,
    'force_deactivation' => false,
    'description'        => __('Drag and drop page builder for WordPress. No programming knowledge required.', 'orbital_station')
),

array(
    'name'               => __('Slider Revolution', 'orbital_station'),
    'slug'               => 'revslider',
    'source'             => get_stylesheet_directory() . '/includes/plugins/revslider.zip',
    'required'           => false,
    'version'            => '5.0.7',
    'force_deactivation' => false,
    'description'        => __('Innovative, responsive WordPress Slider Plugin that displays your content the beautiful way.', 'orbital_station')
),
array(
    'name'              => 'Envato Market',
    'slug'              => 'envato-market',
    'source'            => 'https://envato.github.io/wp-envato-market/dist/envato-market.zip',
    'required'          => true,
    'recommended'       => true,
    'force_activation'  => true,
    'force_deactivation' => true,
),
array(
    'name'      => 'Contact Form 7',
    'slug'      => 'contact-form-7',
    'required'  => false,
    'description'   =>  __('Just another contact form plugin. Simple but flexible.', 'orbital_station')
),

array(
    'name'      => __('WooCommerce - excelling eCommerce', 'orbital_station'),
    'slug'      => 'woocommerce',
    'required'  => false,
    'version'   => '',
    'description'   =>  __('Powerful, extendable eCommerce plugin that helps you sell anything. Beautifully.', 'orbital_station')
),