Add support for clickable hover text or a per pt onclick attribute

Users can put links inside hover labels, but when user try to click the link, the hover label obviously disappears. So perhaps we could add optional “sticky” hover labels, where you click and the hover label is persistent until you click again?

Defining what sticky means will be tricky. It probably doesn't apply to every situation. Maybe we could add it as a new layout.hovermode e.g. 'hovermode: 'sticky'.

So, maybe instead we could define a linkonclick attribute e.g. linkonclick: ['https://plot.ly/~empet/101', 'https://plot.ly', /* ... one link per data pt */] that way clicking on a data pt would open the link instead of hovering on the data pt -> move mouse over link in hover label -> click on it.

cc @jackparmer

未确定文件、测试或入口点。首先明确预期行为是固定显示的悬停标签，还是每个点在点击时对应的链接；只有在交互定义确定且实现与其一致后，才算完成。