Support for interactive Plotly figures when possible

[LaurentRDC]

Based on a comment in #4 :

Pandoc filters only work on the intermediate document representation.

As I see here, getting the output format it possible (I am not sure, because I don't know Haskell)

The idea is to support interactive Plotly plots (via HTML), but fall back to static images when the output format is not HTML, (e.g. PDF)

From Pandoc toJSONFilter documentation:

An alternative is to use the type Maybe Format -> a -> a. This is appropriate when the first argument of the script (if present) will be the target format, and allows scripts to behave differently depending on the target format. The pandoc executable automatically provides the target format as argument when scripts are called using the --filter option.