Move constants to config, update string formatting

Not sure if this is the Elixir-y way to do this, but seems more logical
than hardcoding values such as redis connection.

Also went through and improved how string formatting was performed
throughout the app. Rather than "combining" <> "strings" this way, I'm
now just doing "#${variable}#{formatting}", which looks a lot cleaner.

config/config.exs

@@ -0,0 +1,7 @@
+import Config 
+
+config :privacy_revolver,
+  redis_conn: "redis://localhost:6379",
+  fallback_str: "-fallback",
+  update_file: ".update-results",
+  services_json: "services.json"