Add premium fields to Guild and Member (#656)

* adds premium fields to Guild and Member

* fixes formatting

endpoints.go

@@ -38,6 +38,7 @@ var (
 	EndpointCDNIcons        = EndpointCDN + "icons/"
 	EndpointCDNSplashes     = EndpointCDN + "splashes/"
 	EndpointCDNChannelIcons = EndpointCDN + "channel-icons/"
+	EndpointCDNBanners      = EndpointCDN + "banners/"
 
 	EndpointAuth           = EndpointAPI + "auth/"
 	EndpointLogin          = EndpointAuth + "login"
@@ -97,6 +98,7 @@ var (
 	EndpointGuildAuditLogs       = func(gID string) string { return EndpointGuilds + gID + "/audit-logs" }
 	EndpointGuildEmojis          = func(gID string) string { return EndpointGuilds + gID + "/emojis" }
 	EndpointGuildEmoji           = func(gID, eID string) string { return EndpointGuilds + gID + "/emojis/" + eID }
+	EndpointGuildBanner          = func(gID, hash string) string { return EndpointCDNBanners + gID + "/" + hash + ".png" }
 
 	EndpointChannel                   = func(cID string) string { return EndpointChannels + cID }
 	EndpointChannelPermissions        = func(cID string) string { return EndpointChannels + cID + "/permissions" }

structs.go

@@ -342,6 +342,17 @@ const (
 	MfaLevelElevated
 )
 
+// PremiumTier type definition
+type PremiumTier int
+
+// Constants for PremiumTier levels from 0 to 3 inclusive
+const (
+	PremiumTierNone PremiumTier = iota
+	PremiumTier1
+	PremiumTier2
+	PremiumTier3
+)
+
 // A Guild holds all data related to a specific Discord Guild.  Guilds are also
 // sometimes referred to as Servers in the Discord client.
 type Guild struct {
@@ -446,6 +457,21 @@ type Guild struct {
 
 	// The Channel ID to which system messages are sent (eg join and leave messages)
 	SystemChannelID string `json:"system_channel_id"`
+
+	// the vanity url code for the guild
+	VanityURLCode string `json:"vanity_url_code"`
+
+	// the description for the guild
+	Description string `json:"description"`
+
+	// The hash of the guild's banner
+	Banner string `json:"banner"`
+
+	// The premium tier of the guild
+	PremiumTier PremiumTier `json:"premium_tier"`
+
+	// The total number of users currently boosting this server
+	PremiumSubscriptionCount int `json:"premium_subscription_count"`
 }
 
 // A UserGuild holds a brief version of a Guild
@@ -620,6 +646,9 @@ type Member struct {
 
 	// A list of IDs of the roles which are possessed by the member.
 	Roles []string `json:"roles"`
+
+	// When the user used their Nitro boost on the server
+	PremiumSince Timestamp `json:"premium_since"`
 }
 
 // Mention creates a member mention