Added support to configure a title extractor per file extension

[andresm]

Before this update, Deft used the first line of a file and the
function bound to deft-parse-title-function to obtain the file title
to be displayed.

This update renames deft-parse-title-function to
deft-parse-title-functions. The variable is not bound anymore to a
function used to parse the first line of the files. It is now an
alist that defines the parsers used per file extension. The previous
default parser (deft-strip-title) is maintained as the default
parser. It is associated with :default in the alist.

An example of an .emacs configuration to use Deft and Org-Roam to
maintain titles different from filenames includes:

(defun my-deft-org-title (contents)
"Look for the title in the first 500 characters of an org file.
This function looks for the TITLE property in the first 500
characters of CONTENTS."
(let ((prelude (substring contents 0 (min (length contents) 500))))
(when prelude
(let ((title (substring prelude (string-match "^#\+TITLE:.+$" prelude) (match-end 0))))
(deft-chomp (replace-regexp-in-string "^#\+TITLE:" ""
title))))))

(setq deft-parse-title-functions (push '("org" . my-deft-org-title) deft-parse-title-functions))

[Accacio]

This is great, hope it gets merged