Change: Options to only default when undefined

The input parameters for the Modelo constructors used to default
to an empty object if the given value was falsey. Now it will only
default when the given value is undefined. This allows constructors
to take in primitive values like 'false' or 'null'.

Signed-off-by: Kevin Conway <kevinjacobconway@gmail.com>
Merge pull request #7 from kevinconway/more_options

Change: Options to only default when undefined