`geom_area_ridges`: support variadic `color`

[ASmirnov-HORIS]

Example:

data = {
    'x': [0, 0, 1, 0, 1, 1],
    'g': ['a'] * 3 + ['b'] * 3,
}
ggplot(data, aes('x', 'g')) + geom_area_ridges(aes(color='..y..'), size=2)

Output:

Expected: