Allow commands to return a Task<RuntimeResult> (#466)

* Allow commands to return a Task<RuntimeResult>

This allows bot developers to centralize command result logic by
using result data whether the command as successful or not.

Example usage:
```csharp
var _result = await Commands.ExecuteAsync(context, argPos);
if (_result is RuntimeResult result)
{
    await message.Channel.SendMessageAsync(result.Reason);
}
else if (!_result.IsSuccess)
{
    // Previous error handling
}
```

The RuntimeResult class can be subclassed too, for example:
```csharp
var _result = await Commands.ExecuteAsync(context, argPos);
if (_result is MySubclassedResult result)
{
    var builder = new EmbedBuilder();
    for (var pair in result.Data)
    {
        builder.AddField(pair.Key, pair.Value, true);
    }
    await message.Channel.SendMessageAsync("", embed: builder);
}
else if (_result is RuntimeResult result)
{
    await message.Channel.SendMessageAsync(result.Reason);
}
else if (!_result.IsSuccess)
{
    // Previous error handling
}
```

* Make RuntimeResult's ctor protected

* Make RuntimeResult abstract

It never really made sense to have it instantiable in the first place,
frankly.

src/Discord.Net.Commands/Results/RuntimeResult.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Text;
+
+namespace Discord.Commands
+{
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public abstract class RuntimeResult : IResult
+    {
+        protected RuntimeResult(CommandError? error, string reason)
+        {
+            Error = error;
+            Reason = reason;
+        }
+
+        public CommandError? Error { get; }
+        public string Reason { get; }
+
+        public bool IsSuccess => !Error.HasValue;
+
+        string IResult.ErrorReason => Reason;
+
+        public override string ToString() => Reason ?? (IsSuccess ? "Successful" : "Unsuccessful");
+        private string DebuggerDisplay => IsSuccess ? $"Success: {Reason ?? "No Reason"}" : $"{Error}: {Reason}";
+    }
+}