Add ability to provide custom links to module provider page

It would be good to be able to display custom links on the module provider page. These could be configured globally and use placeholders to get module provider details, such as:

[
  {
    "link_template": "http://github.com/{namespace}/{module}-{provider}/issues",
    "link_text": "Github Issues",
    // Limit to list of namespaces
    "namespaces": ["mynamespace"]
  },
  {
    "link_template": "http://github.com/{namespace}/{module}-{provider}/issues",
    // Use placeholders in link text
    "link_text": "More about {module}",
    // Default to all namespaces
  }
]