luci-app-statistics: add noavg option

It is sometimes useful not to display avg values int the graph.
This option is used to prevent this from appearing in the graph.
In a graph definition this option must be set to *true*,
 if the avg values should not be displayed.

```
options = {
	gauge = {
		title = "Status",
		color = "0000ff",
		noarea = true,
		noavg = true
	}
```

Signed-off-by: Florian Eckert <fe@dev.tdt.de>