Add cache option :default_key_generator to change the key generator at compile time

[cabol]

When using declarative caching, by default, the key generator is set to Nebulex.Caching.SimpleKeyGenerator. But we should be able to change it at compile time with the option :default_key_generator, like so:

defmodule MyApp.Cache do
  use Nebulex.Cache,
    otp_app: :my_app,
    adapter: Nebulex.Adapters.Local,
    default_key_generator: MyApp.Cache.CustomKeyGenerator
end

This will allow changing the default key generator globally avoiding passing it on every caching annotation. However, the annotation :key_generator option remains and can be used at any time to override the default generator.

The suggestion came from issue #122