Add EnumOption with support for proper enum types

Add EnumOption with support for proper enum types

Generalize EnumIntOption such that it can now store the output into a
proper enum type, making it easier to write code where the compiler can
check for correct usage of the enums (i.e., give an error for
unintentional mixing of enum types, or for implicit int-to-enum casts).

Change-Id: Idcdd671f555327a3e3dad521b92d50930cb6e281