Refactor search engines code to be more generic

[asfaltboy]

Make it easier to add new search engines

[asfaltboy]

Basically we need to:

• [ ] Commonize settings for each provider (such as "base_url", "sort_qs", "limit_qs", etc.)
• [ ] Loop over all "stored/activated" search providers.
• [ ] Use the fields from above when creating each search request.

• [ ] Perhaps store all providers in storage/indexedDB api so in the future we can:

  a. Allow user to add his/her own search provider (providing the values for the fields above) b. Allow syncing of this data across google accounts and perhaps push it to a publicly available server (phase2)