Customize slack channel

[putermancer]

This PR adds support for configuring the destination for Slack messages.

1. You can set 'channel' => '#mychannel' or 'channel' => '@myuser' in config.pl to override the destination configured in the Slack webhook
   • Not providing this value at all (or setting it to an empty string) will use whatever is configured in the Slack webhook.
2. You can pass --slack_channel=#mychannel or --slack_channel=@myuser from the command line to override the config setting.
   • This can be useful, for example, to send each user direct messages with their playback notifications, e.g. --notify --user=jack --slack_channel=@jack