Enable cmap option in matplotlib HexTiles

[poplarShift]

Now, the following is possible, whereas before it wasn't:

import holoviews as hv
hv.extension('matplotlib')
import numpy as np
hv.HexTiles(np.random.randn(10000,2)).opts(cmap='Greens', colorbar=True)

If there is any testing being done on cmap and the like, I'd be happy to implement a test if you'd like.

[philippjfr]

Looks like the CI setup is currently broken. I'll try to look into that soon.