make the function to extract a title from a note customizable

[maw]

Background:

• I'm experimenting with note formats that put the title somewhere other than in the first line
• deft-parse-title calls the customizable deft-parse-title-function which defaults to deft-strip-title, but
• deft-parse-title-function is only passed the first line of the note in question.

I have some code (PR coming soon) that creates zetteldeft--title-parsing-function zetteldeft-title-parsing-function. It defaults to deft-parse-title but it lets you replace it with a function that looks like this:

(defun my-title-parsing-function (file contents)
  (let ((title (hypothetical-code-that-uses file contents)))
    (if title
        title
      ;; still need a fallback -- I have a lot of notes in older formats
      (deft-parse-title file contents))))