added 'lower_head', 'upper_head' to bokeh ErrorBars plot (#2227)
added 'lower_head', 'upper_head' to bokeh ErrorBars plot (#2227)


ioam/holoviews,ioam/holoviews,basnijholt/holoviews,basnijholt/holoviews,basnijholt/holoviews,ioam/holoviews