[emacs mode] add tern-highlight-refs command

Hi Marijn,

This is a command `tern-highlight-refs` that lets you highlight occurrences of identifier/property in a given scope in current buffer.

One might bind this command to `mouse-1`, for example, so that it behaves similarly to Sublime Text:

    (add-hook 'tern-mode
              (lambda ()
                (local-set-key [mouse-1] 'tern-highlight-refs))

I'm unsure if `:silent` mode would work here in all cases (as opposed to `:full-file`). Will it be OK, assuming we only want to highlight references in the visible portion of the current buffer?