on-the-fly interactive labels

After implementing support for [streaming data](https://github.com/ContextLab/hypertools/issues/101), it would be nice to be able to interactively label plot points. The way I'm imagining this working is that, when visualizing a data stream, the user might press spacebar, prompting a dialogue box to ask for a label for whatever points are active at that moment (i.e. the current "event"). The user would then type an event label, which would be continually marked on the plot.