Add Slack Notification support

[AmansRevenger]

Basically the title, it's the only thing missing from the original plexrequest-meteor I can think of/ notice missing.

[tidusjar]

The question is, does anyone actually use it? I'm not going to bother implementing it, if no one wants it.

[mwill945]

It's the only thing keeping me from switching over from the meteor version. Both Pushbullet and Pushover fail to send notifications to me on a regular basis, whereas Slack always seems to work.

[tidusjar]

Ok, we will add slack notifications in!

[tidusjar]

@AmansRevender @mwill945 with the meteor version, do you have to manually create an Incoming WebHook ?