Feature Request: Allow user to add arbitrary Searx instance

[Spaghetto392]

I self host a SearxNG instance as a way to configure it all to my liking and (mostly) skirt around instance blocking by search engines. My instance is hosted on the LAN and is not available to the open internet, so it doesn't make sense to add it as an individual instance to the extension.

I believe it would be useful if users may supply an arbitrary address for Searx when setting uBlacklist up, so that they may user their own private instances, should they desire to.