Add option to not cache empty values #15
Description
Currently, the first time a key is looked up by copycat, and it doesn't find the key in the DB, it cascades the lookup to Rails (en.yml), and then always creates a new entry in the DB for that key with whatever value was in the yml. If there was no value it still creates a key with a blank value. That means if somebody later adds a value to the yml it will not make it to the copycat table because subsequent lookups will find the key in the db and never cascade to the yml lookup.
Add a configuration option to not cache keys with no values. That way if a developer adds the key to the template and renders the page before writing the value in en.yml, it won't prevent them from seeding the values later when they add values to the yml.