Add general statistics and visualisations

[alwil]

• [ ] stats boxes in the intro tab
• [ ] top N words in data overview tab
• [ ] evolution of top words in data overview tab

[cforgaci]

@Selkubi working on this, @cforgaci will take over afterwards

[cforgaci]

@alwil pls add examples of stat boxes here

[alwil]

Please look at #150

[Selkubi]

I have added both top N words and evolution of words plots to the shiny app. Please check #139 to move forward with this issue.

[cforgaci]

I have added both top N words and evolution of words plots to the shiny app. Please check #139 to move forward with this issue.

Thanks, @Selkubi! I will pick this up again in a couple of weeks and follow up on #139.