Add ability to customise the SearXNG search engines used for the queries

[felladrin]

Suggestion from a Reddit user [1].

It doesn't seem to provide any ability to customise the search engines used for the queries, though?

[felladrin]

A second Reddit user also commented about this idea [1]:

That's really cool, but it would also be great if there was an option to at least change search providers.
Among other things i would like to see YaCy support. Startpage and Duckduckgo would also be great.