Feature Request: /playnow & /playnext

[download]: https://github.com/jagrosh/MusicBot/releases/latest
[release]: https://img.shields.io/github/release/jagrosh/MusicBot.svg
[setup]: https://github.com/jagrosh/MusicBot/wiki/Setup
[rpi]: https://github.com/jagrosh/MusicBot/wiki/JMusicBot-on-Raspberry-Pi
[features]: https://github.com/jagrosh/MusicBot/projects/1

## General Troubleshooting

Please check off these steps before suggesting a feature or reporting a bug:

- [x] I am running the [latest version][download]: [![Release][release]][download]
- [x] I have followed all instructions on the [setup page][setup] (and if applicable, [Raspberry Pi][rpi])
- [x] I have read through the [planned and suggested features][features]

## Issue

### Issue Type

- [ ] Bug Report
- [x] Feature Request

### Description

The following commands:
* `/playnow ` forces the bot to play the specified song immediately without affecting the queue
* `/playnext ` forces the bot to put a song as the next item in the queue

The issue names the /playnow and /playnext command entry points but no files or tests. Start by tracing existing command handling and queue/playback behavior; done means both commands play the requested URL immediately or place it next without otherwise changing the queue.

Written by the indexing model from the issue text.