allow html writing of waveform plots

This MR would allow the waveform plot to be saved as an interactive HTML using plotly. See an example here.

This is useful for the time domain waveform where it's good to be able to zoom in on the merger, but also plot the inspiral for long signals.

Clicking on the legend also allows lines to be added/removed so we could add more lines, e.g., max likelihood max posterior, etc. if desired.

I think it may even be possible to add a button to switch between time/frequency domain and whitened/unwhitened data (here), but I shouldn't do that now...

The previous behaviour is maintained if the requested format isn't HTML.

pngs can be downloaded from the HTML plot.

I've added plotly to the optional_requirements, but if we want to use this, it may be worth just adding it to the requirements.