[5.3] Added username, icon and channel options for Slack Notifications

[kalfheim]

This PR allows you to set a custom username, icon (emoji) and channel override for incoming webhooks in Slack.

This is a totally supported feature in Slack, so why not take advantage of it?

<?php

class FooNotification extends Notification
{
    public function toSlack($notifiable)
    {
        return (new SlackMessage)
                    ->from('Ghostbot', ':ghost:')
                    ->to('#testytest') // `@username` is also supported
                    ->content('This is a message from a ghost posted in #testytest!');
    }
}

.. will give you this:

As the new methods are optional, this PR is not a breaking change.

Thoughts?

Edit: Renamed methods

[kalfheim]

I guess as cannot be used as a method name 😭 Ideas, anyone?

[crynobone]

@kalfheim from('Ghostbox', ':ghost:')?

[kalfheim]

@crynobone Yup! Sounds good to me.

I also renamed in to to. Makes more sense when sending a notification to a user instead of a channel (by typing @username as opposed to #some_channel)

[taylorotwell]

Has merge conflicts.

[kalfheim]

Thanks, @taylorotwell! Was just about to try and fix the merge conflict, but you beat me to it. 😄

[hinaloe]

Why can't I use icon_url not icon_emoji ?