reading_time: influence of readability

[rgoubet]

As I understand, it seems that reading_time returns a value that in only a function of the word count and a value of milliseconds by character.

However, reading time should also be influenced by the readability score. If there any meaningful way to include a readability weighting to the output of reading_time, e.g. how much longer it takes to read a difficult text (30-49 Flesch grading) than an easy text (80-89 grading)?