eplace DM/Guild preconditions with context

The new RequireContextAttribute works just like RequireDM/RequireGuild,
but is more powerful as developers can specify multiple 'contexts' for
their command to require using the ContextType flags.

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

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Discord.Commands
+{
+    [Flags]
+    public enum ContextType
+    {
+        Invalid = 0, // 00
+        Guild = 1, // 01
+        DM = 2 // 10
+    }
+
+
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+    public class RequireContextAttribute : PreconditionAttribute
+    {
+        public ContextType Context { get; set; }
+
+        public RequireContextAttribute(ContextType context)
+        {
+            Context = context;
+
+            if (Context == ContextType.Invalid)
+                throw new ArgumentException("Context must be a bitfield of ContextType.Guild and ContextType.DM", "context");
+        }
+
+        public override Task<PreconditionResult> CheckPermissions(IMessage context, Command executingCommand, object moduleInstance)
+        {
+            var validContext = false;
+
+            if (Context == ContextType.Invalid)
+                throw new InvalidOperationException("Invalid ContextType");
+
+            if (Context.HasFlag(ContextType.Guild))
+                validContext = validContext || context.Channel is IGuildChannel;
+
+            if (Context.HasFlag(ContextType.DM))
+                validContext = validContext || context.Channel is IDMChannel;
+
+            if (validContext)
+                return Task.FromResult(PreconditionResult.FromSuccess());
+            else
+                return Task.FromResult(PreconditionResult.FromError($"Invalid context for command; accepted contexts: {Context}"));
+        }
+    }
+}