Implement theming for plotly figures

[jackparmer]

copy a minimal implementation in MATLAB from the Python implementation: https://plotly.com/python/templates/

at a minimum, it would be nice to have plotly, plotly_dark, and seaborn themes available (maybe rename them to shorter versions - "grey", "dark", and "seaborn")

could be a new arg in fig2plotly():

f = fig2plot(gcf, theme='dark')

and/or its own function

f2 = theme(f, 'grey')

[jackparmer]

closed via #329