Feature request: Opt out mechanisms

[slatian]

Please make it easy for your users to implement the following opt out mechanisms:

• robots.txt
• deviantarts noai tag
• the w3m TDM protocol (see #124)

With github actively recommending you and 21K+ stars you have a certain kind of responsibility. This is a matter of respect towards the people whose data your users rely on.

One line mentions in the docs I needed grep to find aren't enough.