Support bokeh themes

[jlstevens]

It would be good to support bokeh's theme feature which I don't think we currently do.

One issue is that the documented way of applying a theme is to use doc.theme = Theme(...) and it isn't clear how that would be applied in holoviews.

[philippjfr]

This has been implemented and also documented.

[kcpevey]

@philippjfr Would you mind linking to the docs? I can't seem to find this via searching.

[jlstevens]

@kcpevey The website is due to be updated soon for the 1.10 release. The docs about theming are currently only in the Plotting with Bokeh notebook which you should be able to view on GitHub.