Add RequireBotPermission, rename RequirePermission

This is a breaking change.

Adds a precondition that requires the bot has a specified permission (Resolves #211).

Renames RequirePermission to RequireUserPermission.

src/Discord.Net.Commands/Attributes/Preconditions/RequireBotPermissionAttribute.cs

@@ -0,0 +1,54 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Discord.Commands
+{
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+    public class RequireBotPermissionAttribute : PreconditionAttribute
+    {
+        public GuildPermission? GuildPermission { get; }
+        public ChannelPermission? ChannelPermission { get; }
+
+        public RequireBotPermissionAttribute(GuildPermission permission)
+        {
+            GuildPermission = permission;
+            ChannelPermission = null;
+        }
+        public RequireBotPermissionAttribute(ChannelPermission permission)
+        {
+            ChannelPermission = permission;
+            GuildPermission = null;
+        }
+
+        public override async Task<PreconditionResult> CheckPermissions(CommandContext context, CommandInfo command, IDependencyMap map)
+        {
+            var guildUser = await context.Guild.GetCurrentUserAsync();
+
+            if (GuildPermission.HasValue)
+            {
+                if (guildUser == null)
+                    return PreconditionResult.FromError("Command must be used in a guild channel");
+                if (!guildUser.GuildPermissions.Has(GuildPermission.Value))
+                    return PreconditionResult.FromError($"Command requires guild permission {GuildPermission.Value}");
+            }
+
+            if (ChannelPermission.HasValue)
+            {
+                var guildChannel = context.Channel as IGuildChannel;
+
+                ChannelPermissions perms;
+                if (guildChannel != null)
+                    perms = guildUser.GetPermissions(guildChannel);
+                else
+                    perms = ChannelPermissions.All(guildChannel);
+
+                if (!perms.Has(ChannelPermission.Value))
+                    return PreconditionResult.FromError($"Command requires channel permission {ChannelPermission.Value}");
+            }
+
+            return PreconditionResult.FromSuccess();
+        }
+    }
+}

src/Discord.Net.Commands/Attributes/Preconditions/RequireUserPermissionAttribute.cs

@@ -4,17 +4,17 @@ using System.Threading.Tasks;
 namespace Discord.Commands
 {
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
-    public class RequirePermissionAttribute : PreconditionAttribute
+    public class RequireUserPermissionAttribute : PreconditionAttribute
     {
         public GuildPermission? GuildPermission { get; }
         public ChannelPermission? ChannelPermission { get; }
 
-        public RequirePermissionAttribute(GuildPermission permission)
+        public RequireUserPermissionAttribute(GuildPermission permission)
         {
             GuildPermission = permission;
             ChannelPermission = null;
         }
-        public RequirePermissionAttribute(ChannelPermission permission)
+        public RequireUserPermissionAttribute(ChannelPermission permission)
         {
             ChannelPermission = permission;
             GuildPermission = null;