MAVGraph: Add option to choose plot background colours from active flight mode

Simple little beautification to make it easier to visualise the effect of changing flight modes by colouring the mavgraph background according to the active flight mode at the time.  Uses the same colour pallet as MAVProxy's mavflightview which means it's currently sort of APM-specific (unknown modes will be rendered grey). ![Example output](https://cloud.githubusercontent.com/assets/15090/2748221/a53cb326-c7b0-11e3-82e9-ba2c03af3ae9.png)

At the moment this cannot be specified at the same time as --xaxis as for that, background vregions don't make much sense. You would have to track a per-marker mode/colour instead, and while that's possible, it seems overkill at this stage.