Make `citar-open-entry` configurable

[bdarcus]

Instead of only opening entries in bibtex and such files, also allow Zotero, (ebib?), etc.

Would require:

1. changing citar--open-entry to citar--open-entry-in-file
2. adding a defcustom for the function symbol, which defaults to 1
3. modifying citar-open-entry to use 2
4. adding a function to open zotero select links:

(defun citar-open-entry-in-zotero (citekey)
  "Open a reference item in Zotero/Better BibTeX.

This function assumes a setup where the bibliography files are 
maintained in Zotero, including the citekeys."
  (citar-file-open-external
   (concat "zotero://select/items/@" citekey)))