Ability to change GeoJSON feature styles from the layer itself

[pbugnion]

At the moment, to modify the style of every GeoJSON feature, you have to iterate over all of them:

for feature in layer.features:
    feature.stroke_weight = 4.0

It'd be good to move some of the logic from geojson_layer into the widget. The layer.stroke_weight traitlet should either take a single value or a list and apply the result to its underlying features as necessary.

See issue #228 for how this issue came about.