New verification level

[nickname76]

I think it's too small for pull request, so, please, add new verification level
Like this (discordgo/structs.go):

// Constants for VerificationLevel levels from 0 to 3 inclusive
const (
	VerificationLevelNone VerificationLevel = iota
	VerificationLevelLow
	VerificationLevelMedium
	VerificationLevelHigh
)

// Constants for VerificationLevel levels from 0 to 4 inclusive
const (
	VerificationLevelNone VerificationLevel = iota
	VerificationLevelLow
	VerificationLevelMedium
	VerificationLevelHigh
        VerificationLevelVeryHigh
)

[Bios-Marcel]

Nothing is too small for a pullrequest :P

[Hinara]

Just don't forget to make it on develop ;) But yes small commits are good because they can be revert easily in case of trouble :)