Detect DuckDuckGo browser on desktop and iOS

[adriaandotcom]

Add this for iOS:

https://github.com/duckduckgo/iOS/blob/28f2c58201a6b74abba12fc3ace057d76322b135/Core/UserAgentManager.swift#L99-L103

And this for desktop:

window.navigator._duckduckgoloader_

[adriaandotcom]

Fixed in #34