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/CommandMatch.cs

@@ -20,9 +20,9 @@ namespace Discord.Commands
             => Command.CheckPreconditionsAsync(context, services);
         public Task<ParseResult> ParseAsync(ICommandContext context, SearchResult searchResult, PreconditionResult preconditionResult = null, IServiceProvider services = null)
             => Command.ParseAsync(context, Alias.Length, searchResult, preconditionResult, services);
-        public Task<ExecuteResult> ExecuteAsync(ICommandContext context, IEnumerable<object> argList, IEnumerable<object> paramList, IServiceProvider services)
+        public Task<IResult> ExecuteAsync(ICommandContext context, IEnumerable<object> argList, IEnumerable<object> paramList, IServiceProvider services)
             => Command.ExecuteAsync(context, argList, paramList, services);
-        public Task<ExecuteResult> ExecuteAsync(ICommandContext context, ParseResult parseResult, IServiceProvider services)
+        public Task<IResult> ExecuteAsync(ICommandContext context, ParseResult parseResult, IServiceProvider services)
             => Command.ExecuteAsync(context, parseResult, services);
     }
 }