Implement client interface for elink

[jsstevenson]

Is your feature request related to a problem? Please describe.
I think it's technically possible to get some cross-referential information in more roundabout ways, but elink is helpful for eg hopping between PMIDs and PMC IDs.

Describe the solution you'd like
Add an elink() method onto eutils.client.Client, a la the other public methods on that class. It looks like there's already an elink search method on the QueryService class, it just needs to be plugged into the client.

Describe alternatives you've considered
n/a

Additional context
n/a