Add percentage to Bokeh Donut Chart
<p>I am using Bokeh Donut charts to create pie charts.  I am using the hover tool to show the raw size and category of each slice of the pie on mouse over.  I would also like the hover tool to show the percent of the whole of each slice.  </p>

<p>Is this possible?  My understanding is that there is currently an open pull request focused on building the capability to add arbitrary additional info to the hover tool, but is there any sort of current work around that would allow me to achieve this?  I've been trying to experiment with ways to manually add columns to the hover tool data source (ColumnDataSource?) with no success yet.  Currently the <code>Percentage</code> tooltip for the hover just comes up <code>???</code>.</p>

<pre><code>#limit data 
tension = rrima.loc[:,['community_tension','tension_pct']]

#analysis of community tensions
tensions = Donut(tension,label='community_tension',values='community_tension',agg='count',
                 plot_height=plot_height,plot_width=plot_width,title='Reported Community Tensions')


hover = HoverTool(point_policy='follow_mouse')
hover.tooltips = [('Survey Responses','@values'),('Percentage','@tension_pct'),('Community Tensions','@community_tension')]
tensions.add_tools(hover)
</code></pre>