Add secure Yahoo and DuckDuckGo search support. Add locale specific social sites support

Fixes https://github.com/Shopify/goreferrer/issues/11

Add rules to match:
- r.search.yahoo.com
- r.duckduckgo.com

Also, add a fuzzy match for locale specific server for social sites without explicit list of domains.
For example, recognize `es.reddit.com` as a social site.

@snormore @mkobetic 
/cc @Shopify/reports