Introduce a ToggleButtons button_color style attribute

[simaoh]

Can we change color and font of the buttons in ToggleButtons?
Should this be added to the ToggleButtonsStyle class in ipywidgets/widgets/widget_selection.py? (as in #1189) Something like this:

from .widget_core import CoreWidget
from .widget_style import Style
from .trait_types import InstanceDict, Color

class ToggleButtonsStyle(Style, CoreWidget):
     """Button style widget."""
     _model_name = Unicode('ToggleButtonsStyleModel').tag(sync=True)
     button_width = Unicode(help="The width of each button.").tag(sync=True)
     button_color = Color('#060482', allow_none=True, help="Color of the button").tag(sync=True)
     font_weight = Unicode(help="Button text font weight.").tag(sync=True)

[pbugnion]

Sounds like a good idea to me!

Are you interested in implementing this? A good place to start is looking at the ButtonStyle implementation and the base style implementation in the JS code.

[Doodledun]

@pbugnion I am at PyconCa Sprint, I am very interested in this issue. I am going to try to work on this one. Thanks for the tips!

[jasongrout]

FIxed by #1834. Thanks again!