Convenience method for setting bit_fields using symbols on column_name

I've been messing around with migrating some models from DataMapper over to Sequel.  Sequel-bit_fields has been super helpful.  

My suggested change more or less adds a similar behavior from the Flag type found in dm-types.  This allows assigning bit_fields by passing symbols into the model's associated column_name/property.  Let me know what you think.
I would also suggest to stick to string interpolation rather then concatenation, since its the default used here
[see](https://github.com/sdepold/sequel-bit_fields/blob/master/lib/sequel/plugins/bit_fields.rb#L123). So this might change to `flag_assign = "#{bit_field_column}="`