DuckDuckGo: Continuation of #363.

Pros:
- No more problems with *_html such as https://duckduckgo.com/?q=duckduckgo+help_html.
    
Cons:
- No plain text version (only affects the API so it's not that big of a deal).
- Triggers with "duckduckgo" and "ddg" only so "zeroclickinfo" on its own won't trigger.