Add support for multiple channels to .join and .leave

This commit will allow you to use .join or .leave with the following syntax:
```
.join channel1 channel2 #channel3 channel4
.leave #channel1 channel2 channel3 #channel4
```
The bot will add # to the start of each channel if it doesn't start with it.

This makes joining or leaving a lot of channels a lot easier, and it doesn't interfere with just using one channel.