[Feature Request] Setting to allow !play to be used by users not in voice channel

Example: `Hey (someone not in voice channel) play me a song!`

Will not work. Adding a new setting here should suffice:
https://github.com/jagrosh/MusicBot/blob/master/src/main/java/com/jagrosh/jmusicbot/commands/MusicCommand.java#L71

I would PR, but currently do not have java development setup to test.

Thanks.

Start in src/main/java/com/jagrosh/jmusicbot/commands/MusicCommand.java around line 71 and inspect how the current voice-channel requirement is enforced and how settings are represented. Trace the !play command path and verify that the new setting controls whether users outside a voice channel can request playback.

Written by the indexing model from the issue text.