Add several configuration options for RAM usage

 * `furthest_neighbors`: Uses N most dissimilar users when updating
   recommendations for a user. Similar to kNN.
 * `recommendations_to_store`: the number of recommendations to store in
   Redis per user. Defaults to 100.

Additionally, the kNN and kFN settings will cause Recommendable to
truncate similarity sets after users have been compared. If kNN is set
to 100 and kFN is set to 50, users will only ever have 150 similarity
values in this set. Big memory optimization for Redis!

Signed-off-by: David Celis <me@davidcel.is>