Adding HTML visualization as an option #19

Our current visualization is based on styling pandas tables. This is fine with short sequences where we need to inspect a few items: table visualization does not fit well with longer texts.

I think we should support two HTML-based variants:

• Barplots: (see here for an example)
• Saliency over text: this is standard approach to visualize importance over long sequences. Although we might need to consider recent work that studies how importance is perceived when users are shown such visualization.