Feature Request - Allow control of strokes for points of line charts

It would be nice if you could configure the points in a line graph to strokes (e.g. you want your points to be hollow circles).

It seems like this should be controlled by CSS, except that right now only the fill color is set based on the data - not the stroke color. If the stroke color was set as well (even if the width is 0), it would allow css to be overridden (e.g. fill: white; stroke-width:4px;) to make the points look "hollow").

Right now the only workaround I found was to manually select the correct nth-child number for .c3-chart-line and reset the color.

Edit: I believe just adding a line [here](https://github.com/c3js/c3/blob/master/src/shape.line.js#L314) and a couple other places should be enough to allow css to cover it