I found this is not very useful especially when we're annotating
concave-shaped objects.

```
labelme data_annotated --labels labels.txt --config
  '{shape_color: manual, label_colors: {person: [0, 0, 255]}, default_shape_color: [255,0,0]}'
```

* Fix #266, #285. 'keep_prev' mode state is visible in Edit menu and switchable there

* Fix #266, #285. 'keep_prev' mode state is visible in Edit menu and switchable there
+ flake8 fixes

Close #271