Add default enclosure path directory

[ispringle]

Currently when you save an enclosure the default path is where ever you're current wording directory is. I pretty regularly will pull up elfeed and not be in the directory I'd like to save enclosures to, which leads to switching directories or rewriting the path when I save enclosures. It'd be nice if there was something like elfeed-default-enclosure-directory that elfeed could use in elfeed--request-enclosure-path and `elfeed--request-enclosure-dir.

Right now I'm overwriting this function in my :config block of the use-package statement, but I think it'd be good for everyone if this was available.

Here's what I'm doing

(defvar elfeed-default-enclosure-directory "~/Podcasts/")
(defun elfeed--request-enclosure-path (fname _path)
    "Ask the user where to save FNAME (default is PATH/FNAME)."
    (let ((fpath (expand-file-name
                  (read-file-name "Save as: " elfeed-default-enclosure-directory nil nil fname) elfeed-default-enclosure-directoryo)))
      (if (file-directory-p fpath)
          (expand-file-name fname fpath)
        fpath)))

It'd be trivial to update that path value to:

(when elfeed-default-enclosure-directory
            elfeed-default-enclosure-directory
            path)

I'd be happy to create the PR, just not sure of procedures or if an issue has to exist first.