citar-key-finder should return key string by default

[bdarcus]

Right now, this function returns a target cons that is only useful in Embark.

Perhaps we should tweak things a bit, so we have:

1. citar--key-finder (this is the current function renamed)
2. citar-embark-key-finder (assuming Move embark code to a separate package #643, this is purely an alias for 1)
3. citar-key-finder (this just returns the key from 1)

So:

(defun citar-key-finder ()
  "Return key-at-point as string."
  (cadr (citar--key-finder)))

(defalias 'citar-embark-key-finder #'citar--key-finder
  "Return key-at-point target for Embark.")