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

Perhaps at this point we want a generic list filter function, as keeping track of `snippets` vs `countrySnippets` vs `searchProviderFilteredSnippets` is starting to give me a headache.

Then our logic could look like (in awful pseduocode):

```
var snippets = allSnippets;
snippets = filter(removeGeolocatedSnippetsInWrongCountryFunction, snippets)
snippets = filter(removeFxAccountSnippets, snippets)
snippets = filter(removeSearchProviderSnippets, snippets)
```

Thoughts?