sfeed_update: an optional POSIX shellscript for updating and merging feed items

What can it do:

* Fetching feeds and processing them in parallel.
* Can detect XML text-encoding and handles text conversion to UTF-8 if needed.
* Filter items or change content by some logic: useful for blocking ads too.
* Merge new items and compare with existing items.
* Sort them by newest first.

This script reads a sfeedrc configuration file. This sfeedrc configuration is
just a shellscript that is sourced (just: . $config).

sfeedrc example file:

	feeds() {
		# feed <name> <feedurl> [basesiteurl] [encoding]
		feed "bitreich" "gopher://bitreich.org/0/news.atom.xml"
		feed "codemadness" "https://www.codemadness.nl/atom.xml"
		feed "xkcd" "https://xkcd.com/atom.xml"
	}

To run it:

	sfeed_update

The "database" (so to speak) is the TAB-separated output of sfeed per feed and
is stored for each file by name in the directory ~/.sfeed/feeds/, like:

	~/.sfeed/feeds/bitreich
	~/.sfeed/feeds/codemadness
	~/.sfeed/feeds/xkcd
