Allow other features to designate which content types should receive service links

Debut social integrates the service links module, which includes a setting, service_links_node_types, which is saved as an array of node types that should receive service links. Currently this configuration is not automatic and must be manually configured by a site admin before any node service links will appear.

This is a variable that should not be handled by strongarm, since it will change depending on the install state of the site (e.g., as new features are enabled). To correctly set it, debut_social needs access to data designating the content types that should receive links.

The ability for features to provide settings to other features is a general need in Debut, so I'll open a new issue on Debut.