Add command for customizing current newsticker feed

* lisp/net/newst-backend.el (newsticker-customize-feed): New.
  (newsticker--insert-bytes): Add documentation string.
  (newsticker--decode-iso8601-date): Fix documentation string.
* lisp/net/newst-treeview.el (newsticker-treeview-customize-current-feed):
  New.
  (newsticker-treeview-mode-map): Add key for new command
  'newsticker-treeview-customize-current-feed'. (Fixes second issue in
  Bug#41376.)
3 jobs for master in 12 minutes and 15 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Fast
passed #13558
test-fast-inotify

00:06:28

 
  Normal
passed #13559
test-lisp-inotify

00:05:46

passed #13560
test-lisp-net-inotify

00:05:01