sfeed_update: overriding functions / hooks

The sfeed_update is a shellscript. This way it also allows overriding functions
of this shellscript.

An example by overriding the filter function:

	# filter fields.
	# filter(name)
	filter() {
		case "$1" in
		"bitreich news")
			awk -F '\t' 'BEGIN { OFS = "\t"; }
			tolower($2) ~ /brcon2021/ {
				print $0;
			}';;
		*)
			cat;;
		esac
	}

This filters away all newsitems except "brcon2021" (case-insensitive) in the
title of the feed and merges it with existing items.

It can also be used to block ads, strip parameters, shorten URLs, fixup broken
feeds, write custom connector scripts etc.  There are also practical examples
of this in the README file.
