Feature link names

[pontusarfwedson]

Added functionality to be able to set link_names as parameter in pipeline with it defaulting to false. Also modified the tests so they actually pass with the new added parameter. Has been tested in a pipeline with successfully notified user group by writing @ which then mentioned the group. Was tested by using

resource_types:
- name: slack-notification
  type: docker-image
  source:
    repository: quay.io/pontusarfwedson/slack-notification-resource
    tag: feature-link-names

in our pipeline (which points to a docker image build from this exact branch).

Difference from latest PR is that we now do not have a missing comma in the json in the out file and also the tests has been altered to include the new param.

[drnic]

Thanks!