Add colorbar_title attr to plotly(js) backends

fixes issue #773

src/backends/plotly.jl

@@ -19,7 +19,7 @@ const _plotly_attr = merge_with_base_supported([
     :window_title,
     :guide, :lims, :ticks, :scale, :flip, :rotation,
     :tickfont, :guidefont, :legendfont,
-    :grid, :legend, :colorbar,
+    :grid, :legend, :colorbar, :colorbar_title,
     :marker_z, :fill_z, :levels,
     :ribbon, :quiver,
     :orientation,
@@ -446,6 +446,8 @@ function plotly_series(plt::Plot, series::Series)
         end
     end
 
+    d_out[:colorbar] = KW(:title => sp[:colorbar_title])
+
     clims = sp[:clims]
     if is_2tuple(clims)
         d_out[:zmin], d_out[:zmax] = clims