Add auto-lable (-l) to grdcontour and contour 

**Description of the desired feature**

In GMT 6.1.0, three modules (plot, grdcontour and contour) support the auto-legend feature via **-l** option. 

In PyGMT, currently only the `plot()` function have the `label` argument. We should also add it to `grdcontour` and `contour`.



**Are you willing to help implement and maintain this feature?** Yes, but contributions are welcomed.
