Check for safesearch support

[dalf]

For all engines: an engine which doesn't support safesearch may return NSFW content.

• Query which returns NSFW content, without safesearch disabled.
• Query which returns NSFW content, with safesearch enabled.

If on the first query, there are NSFW content and the engine doesn't support safesearch, issue a error.

If on the second query, there are NSFW content and the engine does support safesearch, issue an error.

About image detection: perhaps use something similar to https://github.com/jivesearch/jivesearch/blob/master/docker/nsfw/Dockerfile