allow custom links e.g. to monitoring tools

[hjacobs]

Adds a new option --links-file to define custom links, e.g. to link to monitoring dashboards or similar:

cluster:
- href: "https://mymonitoringsystem.example.org/dashboard?cluster={name}"
  title: "Grafana dashboard for cluster {name}"
  icon: chart-area
application:
- href: "https://mymonitoringsystem.example.org/dashboard?application={id}"
  title: "Grafana dashboard for application {id}"
  icon: chart-area
- href: "https://apps.mycorp.example.org/apps/{id}"
  title: "Go to detail page of application {id}"
  icon: search
team:
- href: "https://people.mycorp.example.org/search?q=team:{id}"
  title: "Search team {id} on people.mycorp"
  icon: search

[hjacobs]

Fixes #68