Add search operators plugin (#3311)

## What does this PR do?

This PR adds search operator plugin to searx. By default it is disabled because it
removes results from your result set. Thus, you might end up with an empty result page with
the additional filtering.

## Why is this change important?

With all of its shortcomings, still is a nifty plugin.

## How to test this PR locally?

```
batman -site:imdb.com
```

Co-authored-by: DiamondDemon669 <62653580+DiamondDemon669@users.noreply.github.com>