Add GA plugins support

[antonkomarev]

GA plugins description Enhanced Link Attribution plugin Customizing Enhanced Link Attribution

[antonkomarev]

Is 'link.js' optional in string ga('require', 'linkid', 'linkid.js');? In some parts of documentation it's absent.

[antonkomarev]

Enhanced link attribution is implemented.

Syntax with options:

'plugins' => [
    'linkid' => [
        'cookieName' => '_ccli',
        'duration' => 45,
        'levels' => 5,
    ]
]

Syntax without options:

'plugins' => [
    'linkid' => 'linkid.js'
]

Need some examples of custom scripts to implement full plugins support.