VisibilityModifier should be an enum

[jvzijp]

Currently the VisibilityModifier option on the GenerateAutoInterfaceAttribute is of type string. I think it would be better to be able to define this setting using an enum.

Theoretically there could be multiple access modifiers. For example "protected internal:". Therefor the enum should have the [Flags] attribute to be able to define multiple access modifiers or just include all allowed combinations.