[emacs-w3m:13475] w3m-download updates

I've pushed commits to github pull request #27 (download manager, bulk
regex downloads, wget & youtube-dl downloading). Here's a summary for
people on the list who either have been using it or may want to use it.

+ obfuscated-link downloads (ie. those using youtube-dl) are now managed
  by the same queuing system as other downloads, and can be rate-limited
  (ie. throttled).

+ w3m-download-throttle: New defcustom, for rate-limiting.

+ w3m-download-wget-options: New defcustom, for any wget command-line
  feature.

+ Emulate the no-clobber behavior of wget, ie. repeated downloads have
  numeric extensions appended to them.

  + Thus, repeat of completed download is now available.

  + Not yet available for youtube-dl downloads.

+ Only allow a single queue buffer.

+ Tab and back-tab now navigate among entries in the queue buffer.
  (w3m-download-queue-next-entry, w3m-download-queue-previous-entry)

In order to use the features, it should be sufficient to download and
evaluate only file w3m-download.el from the master branch at

   https://github.com/Boruch-Baum/emacs-w3m

That branch has also integrated all my other pull requests to the
project.

Feedback is most welcome, but depending on its nature, in order not to
clutter the mailing list, consider the options of commenting on the
project's github PR #27, or submitting an issue at my repository, or
emailing me off-list.

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