add voat.co engine

Simplify search.py

Sometimes openstreetmap returns with None as title. In these cases use an empty
string instead.

closes #758

set search language from settings.yml

also tried flask's flash feature but flask creates session cookies if it
isn't flushed. Avoiding session cookies to preserve privacy

closes #756
closes #761

see https://msdn.microsoft.com/en-us/library/ff795620.aspx for bing
specific search operators

closes #755

add lobste.rs xpath engine