Add feed generating capabilities

`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.

@@ -0,0 +22,4 @@
+	Content string // HTML of entry
+}
+
+type User struct {

@@ -0,0 +29,4 @@
+
+type tildeData struct {
+	News        []newsEntry
+	Users       []User

@@ -0,0 +30,4 @@
+type tildeData struct {
+	News        []newsEntry
+	Users       []User
+	ActiveUsers []string `json:"active_users"`

@@ -0,0 +33,4 @@
+	ActiveUsers []string `json:"active_users"`
+}
+
+type ByName []User

@@ -0,0 +35,4 @@
+
+type ByName []User
+
+func (n ByName) Len() int           { return len(n) }

@@ -0,0 +47,4 @@
+
+	type tmplData struct {
+		News   []newsEntry
+		Lights string

@@ -0,0 +52,4 @@
+
+	td := &tmplData{
+		News:   data.News,
+		Lights: "",

@@ -0,0 +55,4 @@
+		Lights: "",
+	}
+
+	sort.Sort(ByName(data.Users))

@@ -0,0 +57,4 @@
+
+	sort.Sort(ByName(data.Users))
+
+	isActive := func(username string) bool {

@@ -0,0 +67,4 @@
+		return false
+	}
+
+	for _, u := range data.Users {