Added option for gradient coloring and alpha in edges

[fransua]

Also a notebook to summarize options

The idea:

    'Ben': 'tab:red',
    'Betty': 'tab:blue',
    'Mike': 'tab:orange',
    'John': 'tab:green',
    'drinks': 'tab:purple',
    'fruit': 'olive',
    'meat': 'tab:brown',
    'bread': 'gold',
    ('John', 'fruit'): 'red'
}

alpha_dict = {
    'Ben': 0.65,
    'Betty': 0.65,
    'Mike': 0.65,
    'John': 0.2,
    'drinks': 0.65,
    'fruit': 0.65,
    'meat': 0.65,
    'bread': 0.65,
    ('John', 'fruit'): 1
}

ax, left_coords, right_coords = sankey(
    left=df['customer'].values[1:], right=df['good'].values[1:],
    rightWeight=weight, leftWeight=weight, aspect=20, fontsize=20, color_gradient=True,
    rightLabels=['bread', 'meat', 'fruit', 'drinks'], colorDict=color_dict, alphaDict=alpha_dict
)