Add an Option to Omit Empty Attributes

This PR addresses #9, providing users with an option to omit empty attributes from their seed files.  In an application with models that have a large number of fields that do not have `nil` constraints, there's the potential for a ton of attribute/value pairs in the seed files that have `nil` values.

The `:omit_empty_attrs` option will strip these out so that the seed files only contain attribute/value pairs with non-`nil` values.