support eldoc-box

[unhammer]

eldoc-box-hover-mode from https://github.com/casouri/eldoc-box needs a configure, maybe something like

(defun dimmer--is-eldoc-box ()
    (equal eldoc-box--buffer (buffer-name)))
  (add-to-list 'dimmer-prevent-dimming-predicates
               #'dimmer--is-eldoc-box)

(also, a new tag would be nice, stable.melpa.org installs one that fails with newer emacsen due to define-obsolete-variable-alias missing the version string)