Expand functionality for "Unknown" values

[MitchellBot]

Great project. Thank you for all your hard work!

I only have one request about handling "Unknown" values.

When creating enums, it's best practice to have a value that represents "Unknown" input. This helps with JSON marshaling and allows creating the object with a default value before setting it (helpful for pflags). For this situation though, the IsValid should return false for whichever value is Unknown. But when using an Unknown value with go-enum, it's treated as IsValid=true and also listed in the error message as a valid option.

Additionally, the integer for the unknown value should be assignable. For instance, I'm working with a backend enum that does not have an Unknown representation but uses index 0 for a valid type. However, even though the backend doesn't use Unknown types, I still wish to use it for the above mentioned reasons. Thus, I have to use -1, like so:

Example:

/* ENUM(
Unknown = -1
OneThing = 0
Something = 18
AnotherThing = 25
) */
type MyType int

This works to create an Unknown representation with value -1, but the Parse function automatically defaults to MyType(0) - whether it exists or not.

To summarize, the desired behavior is:

• if someone tries to parse "YetAnotherThing", it should map to Unknown (-1), not MyType(0) which is OneThing
• when the error prints, it should never say not a valid MyType, try [Unknown, ...] - it should only list the valid types
• If we do parse an invalid value - and it gets mapped to Unknown - then IsValid() should not return true