Add config option to show polygon labels on canvas

[maxkarelov]

There are a new boolean config option shape.show_labels to draw label names over polygons for better visual labelling and default_create_mode for default type of labelling objects.

In our team we have the case where we continuously improve model by using it's predictions on unknown pictures and review it in labelme to speed up labelling process.

Before

After

[wkentaro]

Thank you for the nice feature implementation. I'm happy to include this.

Can you have a look at the below results? Maybe we need to have a better algorithm to define the x,y coordinate of the label?

labelme examples/bbox_detection/data_annotated --config '{"shape": {"show_labels": true}}'