legend_opts implemented

[ahuang11]

#4356

import holoviews as hv
hv.extension('bokeh')

ovl = hv.Curve([0, 1, 2], label='BLUE') * hv.Curve([0, 1, 4], label='RED')
ovl.opts('Overlay', legend_opts={'background_fill_alpha': 0.5, 'background_fill_color': 'red'})

hv.extension('matplotlib')
ovl = hv.Scatter([0, 1, 2], label='BLUE') * hv.Scatter([0, 1, 4], label='RED')
ovl.opts('Overlay', legend_opts={'facecolor': 'red'})

[jlstevens]

Looks good to me! Thanks for the PR!

[philippjfr]

Thanks a lot @ahuang11! A few tests would be appreciated checking that the opts are applied to the models.

[philippjfr]

Thanks again @ahuang11!