Add type for namespaces for better UI

This adds `type Namespaces []Namespace` so that methods can be added to
this slice so that it is easier for consumers to work with the values.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
I think this probably should be `Add` and current `Add` should be private or removed. I don't see case when I don't want to check for existing namespace on `Add`.