Enhanced geom_forecast(series=*) functionality

Now always sets colour according to series, not only when series colour is inherited from ggplot() mapping. (Usually from autoplot.mts)

```R
autoplot(USAccDeaths) + 
    geom_forecast(snaive(USAccDeaths), series="Seasonal Naive", plot.conf = FALSE) + 
    geom_forecast(naive(USAccDeaths, h=24), series="Naive", plot.conf = FALSE) +
    geom_forecast(forecast(ets(USAccDeaths)), series="ETS", plot.conf = FALSE) + 
    guides(colour=guide_legend(title="Forecast"))
```