Add IChannel.Nsfw, RequireNsfw precondition (#633)

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

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

@@ -0,0 +1,20 @@
+using System;
+using System.Threading.Tasks;
+
+namespace Discord.Commands
+{
+    /// <summary>
+    /// Require that the command is invoked in a channel marked NSFW
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+    public class RequireNsfwAttribute : PreconditionAttribute
+    {
+        public override Task<PreconditionResult> CheckPermissions(ICommandContext context, CommandInfo command, IDependencyMap map)
+        {
+            if (context.Channel.IsNsfw)
+                return Task.FromResult(PreconditionResult.FromSuccess());
+            else
+                return Task.FromResult(PreconditionResult.FromError("This command may only be invoked in an NSFW channel."));
+        }
+    }
+}