Accept opaque identifier and full URI

[elf-pavlik]

For example https://github.com/w3c/node-w3capi#users

If I start with w3capi.group(110151).users().fetch() I will get a list of users, now to get affiliations of each user I can't simply iterate with w3capi.user(user.href).affiliations().fetch(), instead I need to extract opaque identifiers from the URI with something like user.href.split('/').pop(). It would be nice if the client checked if URI is being passed and accept them wherever it accepts opaque identifiers.