`feed.tmpl.xml` should be RSS2.0-conforming. Ideally each news item
would have a date of publication, but RSS doesn't require it.
I found it easier to copy `genblog.go` to `genfeed.go` than to shoehorn
in feed logic to the blog generator. As such, `genfeed.go` might have
some unnecessary stuff in it (though the go compiler didn't complain, so
who knows?!). I also added the necessary machinery in
`generate_homepage`.
Of course, I did very minimal testing.
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