Add custom sorting to the twitch channels widget

this allows the channels to be sorted as defined in the config while
still keeping the live channels on the top. the wording could probably
be improved, "custom" is too broad but i'm not sure what else to call
it.

internal/feed/twitch.go

@@ -44,6 +44,12 @@ func (channels TwitchChannels) SortByViewers() {
 	})
 }
 
+func (channels TwitchChannels) SortByLive() {
+	sort.SliceStable(channels, func(i, j int) bool {
+		return channels[i].IsLive && !channels[j].IsLive
+	})
+}
+
 type twitchOperationResponse struct {
 	Data       json.RawMessage
 	Extensions struct {