[Feature Request] Command to searchnext (like playnext)

**Is your feature request related to a problem? Please describe.**
Add a new command that functions like `playnext` except it goes through the `search` workflow.

**Describe the basic flow/steps of using this feature**
User uses the command `searchnext` which follows the `search` workflow except it adds it next in the queue.

**Have you confirmed that this idea is not already described here?**
https://github.com/jagrosh/MusicBot/projects/1
Yes, and it does not appear to have been suggested under Issues yet.

Start by locating the existing `playnext` command and the `search` workflow in the Java source. Trace how each command is registered and how items are added to the queue. Done means a `searchnext` command follows the search flow and adds the selected result next in the queue.

Written by the indexing model from the issue text.