docs v2: colormap helper API

[manzt]

from the prior docs:

Color Maps
-----------

Certain quantative tracks such as the heatmap can vary their colormap. Color maps can be passed in directly as arrays of color values:

.. code-block:: python
    Track('heatmap', tileset, colorRange=['white', 'black'])

Or created from a matplotlib colormap (reversed=True reverses the color order in the heatmap):

.. code-block:: python

    from higlass.utils import hg_cmap
    Track('heatmap', tileset, colorRange=hg_cmap('jet', reverse=True))


A list of available matplotlib color maps can be found [in the matplotlib docs](https://matplotlib.org/3.1.1/gallery/color/colormap_reference.html).

[manzt]

We don't currently have the hg_cmap utility. Should we add it?