Allow custom use agent for specific domain

[ghost]

Hello!
Please add the ability to select a specific user agent for a specific site. This would be useful when the site only allows access from a specific browser.

Here is one example:
https://duckduckgo.com/email/

The site only works with their official browser, but I only want to use my Firefox. In case this feature comes up, you can simply specify the user agent of their official browser for this site.