Fix part() not processing channel name

[a-random-lemurian]

part(channel) is not properly processing channel names before sending PART commands to Twitch, resulting in PARTs failing to go through. This pull request will add the missing call to _.channel() ensuring that client.part('a_random_lemurian') (note the missing #) works just like client.part('#a_random_lemurian').

part(channel) {
    channel = _.channel() // This line of code should have been added
    return this._sendCommand({ delay: null, channel: null, command: `PART ${channel}` }, (res, rej) =>
        this.once('_promisePart', err => !err ? res([ _.channel(channel) ]) : rej(err))
    );
}