Add template for Slack notifier

Added a template to generate the Slack configuration when you specify it on the CLI:
```bash
backup generate:model -t my_backup --notifiers="slack"
```
Should generate:
```ruby
# encoding: utf-8

##
# Backup Generated: my_backup
# Once configured, you can run the backup with the following command:
#
# $ backup perform -t my_backup [-c <path_to_configuration_file>]
#
# For more information about Backup's components, see the documentation at:
# http://meskyanichi.github.io/backup
#
Model.new(:my_backup, 'Description for my_backup') do

  ##
  # Slack [Notifier]
  #
  notify_by Slack do |slack|
    slack.on_success = true
    slack.on_warning = true
    slack.on_failure = true

    # The team name
    slack.team = 'my_team'

    # The integration token
    slack.token = 'xxxxxxxxxxxxxxxxxxxxxxxx'

    ##
    # Optional
    #
    # The channel to which messages will be sent
    # slack.channel = 'my_channel'
    #
    # The username to display along with the notification
    # slack.username = 'my_username'
  end

end
```