Add support for "current" core and profile

Within dslm_base/cores and dslm_base/profiles, it would be nice if we could place a "current" symlink pointing to a single core or profile. New sites could then be created pointing to the "current" link and updates would be more easily rolled out given only a single symlink needs to change.

This should be as simple as adjusting the protected $core_regex and protected $profile_regex regex's.

I'm probably also going to add a --current CLI option (similar to --latest) which simply uses the current symlink if its in place.