[fix bug 1104336] Add option to exclude search engines.

So here's a better reason why we shouldn't use `searchEngineName`: When I set my search engine to Wikipedia, `searchEngineName` is "Wikipedia (en)". That space is going to screw up this data attribute.

Whereas the identifier returned by `getConfiguration` is just `wikipedia`.

I actually think the best thing to do here is to have a single comma-separated exclusion attribute, so that each snippet has something like `data-exclude-search-engines="yahoo,google"`. Then we avoid any issues with malformed identifiers.