contrib/lisp/org-contacts.el: Complete contacts using tags and properties

* contrib/lisp/org-contacts.el (org-contacts-complete-tags-props): New
function that allows the user to retrieve contacts based on tags and
properties.

(org-contacts-complete-functions): Add
org-contacts-complete-tags-props to the completion functions.

(org-contacts-tags-props-prefix): Prefix used to detect the
completion method wished.

Based on the idea and implementation of John Kitchin
contrib/lisp/org-contacts.el: Fix a bug when contacts don't have an email address

* contrib/lisp/org-contacts.el (org-contacts-complete-tags-props,
org-contacts-complete-group): Fix a bug when contacts don't have an
email address.