Allow embedding images/HTML in notices

[MarkusTeufelberger]

At the moment it is only possible to set a title, URL/link and a severity according to https://github.com/Netflix/vizceral/wiki/How-to-Use

I would like to also have a way to embed e.g. a small picture (e.g. a metrics graph) or some longer text elements in the notice.

Reason: It would be nice to also have a way to visualize latency and/or capacity of network links between nodes/servers. While this is probably difficult to do in WebGL in a way that is intuitive, at least displaying existing metrics graphs in there could already help.

[IceFrozen]

maybe they give up the project. @MarkusTeufelberger

[jrsquared]

That would be nice. The notice is just a regular html element (https://github.com/Netflix/vizceral/blob/master/src/notices.js#L53). It shouldn't be too much work to add the ability to embed an image as well. Happy to review a PR to support this.