Add command prefix configuration

PluralKit.Bot/BotConfig.cs

@@ -2,7 +2,14 @@ namespace PluralKit.Bot
 {
     public class BotConfig
     {
+        public static readonly string[] DefaultPrefixes = {"pk;", "pk!"};
+
         public string Token { get; set; }
         public ulong? ClientId { get; set; }
+        
+        // ASP.NET configuration merges arrays with defaults, so we leave this field nullable
+        // and fall back to the separate default array at the use site :)
+        // This does bind [] as null (therefore default) instead of an empty array, but I can live w/ that. 
+        public string[] Prefixes { get; set; }
     }
 }