make colormap an independent dependency

[vincentsarago]

closes #251

This PR tries to simplify how we can register colormaps

from rio_tiler.colormap import cmap

# register new cmap if you want 
# cmap = cmap.register({"above": custom_colormap.above_cmap})

# define a enums with the available cmaps
ColorMapNames = Enum(  # type: ignore
    "ColorMapNames", [(a, a) for a in sorted(cmap.list())]
)

# create a cmap dependency
def ColorMapParams(
    color_map: ColorMapNames = Query(None, description="Colormap name",)
) -> Optional[Dict]:
    """Colormap Dependency."""
    if color_map:
        return cmap.get(color_map.value)
    return None


tiler = TilerFactory(colormap_dependency=ColorMapParams)

To Do

• write doc