Add file/note data to candidates

[bdarcus]

This PR has two commits; both to facilitate programmatic filtering etc of candidates.

Add cons cells to candidate entries

First, when creating candidates, it adds cons cells for files or notes, I can drop this commit if we prefer the more general solution below, but I don't believe the operations here (pushing cons to a list) are expensive.

(("has-note" . t) ; this PR adds this and next line
 ("has-file" . t)
 ("doi" . "10.1111/j.1540-6040.2009.01269.x")
 ("journaltitle" . "City \\& Community")
 ("date" . "2009-03-01")
 ("author" . "Zukin, Sharon and Trujillo, Valerie and Frase, Peter and Jackson, Danielle and Recuber, Tim and Walker, Abraham")
 ("title" . "New Retail Capital and Neighborhood Change: Boutiques and Gentrification in {{New York City}}")
 ("=type=" . "article")
 ("=key=" . "zukin2009"))

To filter:

(seq-filter 
  (lambda (entry) 
    (assoc "has-note" entry)) 
  (citar--get-candidates))

Add filter arg to citar--get-candidates

Second, more generally, it adds an optional filter to citar--get-candidates:

(citar--get-candidates t (citar-has-note))

FWIW, this is actually slower than the first option (though that's not surprising; this is running dynamically, while the former is static cached data).

ELISP> (benchmark-run-compiled 10 (citar--get-candidates nil (citar-has-note)))
(0.036507296 0 0.0)

ELISP> (benchmark-run-compiled 10 (seq-filter 
    (lambda (e) 
      (assoc "has-note" e)) 
  (citar--get-candidates)))
(0.004006728 0 0.0)

See Vidianos-Giannitsis/zetteldesk.el#5

Close #594

[bdarcus]

@localauthor - this may or may not be useful for zk.