Add cformatter for bokeh

[ahuang11]

import xarray as xr
import hvplot.xarray

import holoviews as hv

hv.extension('bokeh')

da = xr.tutorial.open_dataset('air_temperature')['air'].isel(time=0)
da.hvplot('lon', 'lat').opts(clabel='red', cformatter='%.5f')

[philippjfr]

Thanks @ahuang11!