Change `features` fields to be a Vec<String>

When Discord adds new features, the Feature enum will not be able to
serialize the new values until updated, causing the entire Guild
deserialization to fail.

Due to the fact that these features can be added at any time, the
`features` vector on Guild and PartialGuild have been changed to be a
`Vec<String>`.

Upgrade path:

Instead of matching on variants of Feature like so:

```rust
use serenity::model::Feature;

for feature in guild.features {
    if feature == Feature::VipRegions {
        // do work with this info
    }
}
```

Instead opt to check the string name of the feature:

```rust
for feature in guild.features {
    if feature == "VIP_REGIONS" {
        // do work with this info
    }
}
```

3 files changed, 15 insertions, 30 deletions

diff --git a/src/model/guild/feature.rs b/src/model/guild/feature.rs
deleted file mode 100644
index 40fd3fd..0000000
--- a/src/model/guild/feature.rs
+++ /dev/null
@@ -1,25 +0,0 @@
-/// A special feature, such as for VIP guilds, that a [`Guild`] has had granted
-/// to them.
-///
-/// [`Guild`]: struct.Guild.html
-#[derive(Copy, Clone, Debug, Deserialize, Hash, Eq, PartialEq)]
-pub enum Feature {
-    /// The [`Guild`] can set a custom [`splash`][`Guild::splash`] image on
-    /// invite URLs.
-    ///
-    /// [`Guild`]: struct.Guild.html
-    /// [`Guild::splash`]: struct.Guild.html#structfield.splash
-    #[serde(rename = "INVITE_SPLASH")]
-    InviteSplash,
-    /// The [`Guild`] can set a Vanity URL, which is a custom-named permanent
-    /// invite code.
-    ///
-    /// [`Guild`]: struct.Guild.html
-    #[serde(rename = "VANITY_URL")]
-    VanityUrl,
-    /// The [`Guild`] has access to VIP voice channel regions.
-    ///
-    /// [`Guild`]: struct.Guild.html
-    #[serde(rename = "VIP_REGIONS")]
-    VipRegions,
-}
diff --git a/src/model/guild/mod.rs b/src/model/guild/mod.rs
index 110ef4e..3615e5d 100644
--- a/src/model/guild/mod.rs
+++ b/src/model/guild/mod.rs
@@ -1,5 +1,4 @@
 mod emoji;
-mod feature;
 mod guild_id;
 mod integration;
 mod member;
@@ -8,7 +7,6 @@ mod role;
 mod audit_log;
 
 pub use self::emoji::*;
-pub use self::feature::*;
 pub use self::guild_id::*;
 pub use self::integration::*;
 pub use self::member::*;
@@ -62,8 +60,15 @@ pub struct Guild {
     /// VIP features enabled for the guild. Can be obtained through the
     /// [Discord Partnership] website.
     ///
+    /// The following is a list of known features:
+    ///
+    /// - `INVITE_SPLASH`
+    /// - `VANITY_URL`
+    /// - `VERIFIED`
+    /// - `VIP_REGIONS`
+    ///
     /// [Discord Partnership]: https://discordapp.com/partners
-    pub features: Vec<Feature>,
+    pub features: Vec<String>,
     /// The hash of the icon used by the guild.
     ///
     /// In the client, this appears on the guild list on the left-hand side.
@@ -1351,7 +1356,7 @@ impl<'de> Deserialize<'de> for Guild {
             .map_err(DeError::custom)?;
         let features = map.remove("features")
             .ok_or_else(|| DeError::custom("expected guild features"))
-            .and_then(serde_json::from_value::<Vec<Feature>>)
+            .and_then(serde_json::from_value::<Vec<String>>)
             .map_err(DeError::custom)?;
         let icon = match map.remove("icon") {
             Some(v) => Option::<String>::deserialize(v).map_err(DeError::custom)?,
diff --git a/src/model/guild/partial_guild.rs b/src/model/guild/partial_guild.rs
index cb2f8ef..6a8717d 100644
--- a/src/model/guild/partial_guild.rs
+++ b/src/model/guild/partial_guild.rs
@@ -17,7 +17,12 @@ pub struct PartialGuild {
     pub embed_channel_id: Option<ChannelId>,
     pub embed_enabled: bool,
     #[serde(deserialize_with = "deserialize_emojis")] pub emojis: HashMap<EmojiId, Emoji>,
-    pub features: Vec<Feature>,
+    /// Features enabled for the guild.
+    ///
+    /// Refer to [`Guild::features`] for more information.
+    ///
+    /// [`Guild::features`]: struct.Guild.html#structfield.features
+    pub features: Vec<String>,
     pub icon: Option<String>,
     pub mfa_level: u64,
     pub name: String,