Phase 2 resource support: dynamically loadable resource plugins

[knoepfel]

As described in #21, this gives the ability for users to create a translation unit with a registration stanza like:

PHLEX_REGISTER_RESOURCES(r, config)
{
  r.add_resource<T>(...);
  r.add_resource<U>(...);
}

These plugins are loaded before any other plugins in a Phlex program. Included as part of this issue is the need to specify configuration artifacts as to which resource plugins should be loaded for a Phlex job.