Command arguments

[GerardSmit]

This PR implements command arguments.

TODO

• Merge Added Microsoft.Extensions.Hosting support #18.
• Fix the order of the commands. Currently if a non-argument method is above a method with arguments it'll always execute the non-argument method.

Example

Command("greet")]
public Task GreetAsync(IContext e, string name)
{
    return e.GetChannel().SendMessageAsync($"Hello {name}!");
}

[Command("greet")]
public Task GreetAsync(IContext e)
{
    return e.GetChannel().SendMessageAsync("Hello nobody...");
}