do not set cookie or gather referrer on DNT

[alpipego]

if a user has set Do Not Track, don't set a cookie (even if the setting
is enabled) and don't try to get the referrer, fixes #68.

[dannyvankooten]

Hey @alpipego,

I'm afraid I can't merge this right away as it changes the default behavior of the plugin and in turn will effect statistics for people who've been running Koko Analytics for a while now. I also think it's debatable what doNotTrack actually means and what users want it to do, e.g. people might also use it to signal they don't want to waste any unnecessary bandwidth when data is limited (large parts of the world).

What we can do is make this configurable through a filter hook by adding a property to the window.koko_analytics configuration object. Would that work for you?