Make matplotlib a soft dependency of the bokeh backend

[jlstevens]

Currently, you need matplotlib for certain functionality (e.g colormaps) to use bokeh. Even if certain features are missing without matplotlib, I feel the bokeh backend should be usable even without matplotlib installed.

[philippjfr]

Quick note, I think the matplotlib compatibility layer in bokeh is already factored out fairly cleanly, so all that has to happen is wrapping imports in try/excepts and for a small number of functions in holoviews.plotting.bokeh.util to handle the lack of matplotlib gracefully. The other missing piece are the magics, which I think currently assume matplotlib as the default backend, causing all kinds of issues when it's not available. Not a huge job and definitely worth doing.

[philippjfr]

You can now use the bokeh backend without installing matplotlib.