Allow defining color intervals

[philippjfr]

This PR adds support for defining color intervals via the color_levels option. The color_levels option now allows defining a list of levels to map each supplied color to.

The example used in the docs is as follows:

levels = [0, 38, 73, 95, 110, 130, 156, 999]  
colors = ['#5ebaff', '#00faf4', '#ffffcc', '#ffe775', '#ffc140', '#ff8f20', '#ff6060']

path = [
    (-75.1, 23.1, 0),   (-76.2, 23.8, 0),   (-76.9, 25.4, 0),   (-78.4, 26.1, 39),  (-79.6, 26.2, 39),
    (-80.3, 25.9, 39),  (-82.0, 25.1, 74),  (-83.3, 24.6, 74),  (-84.7, 24.4, 96),  (-85.9, 24.8, 111),
    (-87.7, 25.7, 111), (-89.2, 27.2, 131), (-89.6, 29.3, 156), (-89.6, 30.2, 156), (-89.1, 32.6, 131),
    (-88.0, 35.6, 111), (-85.3, 38.6, 96)
]

hv.Path([path], vdims='Wind Speed').options(
    color_index='Wind Speed', color_levels=levels, cmap=colors, line_width=8, colorbar=True, width=450
)

[philippjfr]

Ready to merge when tests pass.

[jlstevens]

Looks good to me. Merging.