Added nanosecond y-scale formatting

Hello devs,
We've recently started using Grafana and really like it (kudos!).

Some of our metrics though are reported in *nanoseconds* - which are not available by default in the y-scale selection, so I've added `ns` to the available formatters. It would be very cool if this got merged, so we don't have to maintain our own branch with this formatter added :-)

Let me know what you think about this - thanks in advance!