Allow custom fields to be passed into the slack_fields payload

[mikespokefire]

Currently, you can only use the built in slack fields of:

['status', 'stage', 'branch', 'revision', 'hosts']

This change allows custom fields to be passed through by defining a
mapping of how to display them. e.g.

set :slack_fields, ['status', 'environment', 'docker_image']
set :slack_custom_field_mapping, -> {
  {
    'environment' => {
      title: 'Environment',
      value: -> {
        if fetch(:stage) == :production
          'production'
        else
          "staging-#{fetch(:staging_name)}"
        end
      },
      short: true,
    },
    'docker_image' => {
      title: 'Docker image',
      value: fetch(:docker_image),
      short: false,
    },
  }
}