Add support for finalize_hooks to bokeh backend

[philippjfr]

We've had finalize_hooks to apply customizations of plots that are currently not directly supported by holoviews in the matplotlib backend for a long time now. This PR adds support for finalize_hooks to bokeh as well.

Here is a simple example that can be used as a workaround for #1039:

def apply_dt_formatter(plot, element):
    plot.handles['xaxis'].formatter = bokeh.models.DatetimeTickFormatter()

drange = pd.date_range(start="2014-01-01", end="2016-01-01")
vals = drange.values.astype('int64')/10**6
hv.Curve((vals, np.arange(len(vals))))(plot=dict(finalize_hooks=[apply_dt_formatter]))

[philippjfr]

hooks aren't called quite in the right place, will fix shortly.

[philippjfr]

Had to rebase to get the HeatMap changes. Ready to merge once it's passing.

[jlstevens]

Tests have passed. Merging.

[thoth291]

Doesn't work for my Linux installation (hv - 1.9.2).

[philippjfr]

Your call back references bokeh but you haven't imported it.

[thoth291]

🤥 OMG! I guess I should stop working on weekends... Sorry for the bothering.... Thanks for your help!!!

[philippjfr]

No worries, I know the feeling 😃

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