Add enum flag type

[parrasajad]

Example:

const (
	Nil goflags.EnumVariable = iota
	Type1
	Type2
)
allowedTypes := goflags.AllowdTypes{"type1": Type1, "type2": Type2}

flagSet.EnumVarP(&options.Type, "enum-type", "et", Nil, "Variable Type (type1/type2)", allowedTypes)

output:

$go run . --type type3
invalid value "type3" for flag -type: allowed values are type1, type2

$go run . --type type1 //ok

[Mzack9999]

lgtm - Remaining todos:

• Tests (both positive and negative)
• Update docs