Add global placeholders and values to be substituted to main configuration file

It's the feature related to Management system for websites pool.

The requirement is to deliver/share the data between the websites in the pool. Consider the case, we have 3 websites:

• site1.com
• site2.net
• site3.org

In main configuration file we have the section:

{
    "placeholders": {
        "first_site_in_pool": {
            "url": "site1.com",
            "anchor": "The first website in our pool",
            "title": "The first website in our pool: Lorem Ipsum",
        },
        "second_site_in_pool": {
            "url": "site2.net",
            "anchor": "The second website in our pool",
            "title": "The second website in our pool: Lorem Ipsum",
        },
        "third_site_in_pool": {
            "url": "site3.org",
            "anchor": "The third website in our pool",
            "title": "The third website in our pool: Lorem Ipsum",
        }
    }
}

We have to be able to use first_site_in_pool_url, third_site_in_pool_title, etc... in RIOT.js components and <TMPL_VAR second_site_in_pool_anchor> in generic templates.