Allow bots to send slash commands

[lucasgmagalhaes]

I'm currently developing a library that tests Discord bots. To do so, I instantiate another bot that sends command messages to the channel so the testing bot does what they have to do. The problem is that as only real users can send slash commands, I'm not able to test slash commands with my library. So it would be very helpful for me and for those who are using my library (corde).