Add "textplot_frequency"

[stefan-mueller]

We could add a new plotting object called, for example, textplot_frequency(), which is based on this plot from the vignette.

On the x-axis we would have the topfeatures, on the y-axis the term frequency. Once the groups option for topfeatures is implemented (see #824), we could group topfeatures by document based on facet_wrap. Let me know if you think it's reasonable to add this plot, and I can make a PR.

[stefan-mueller]

I think we can close this as the manual of textstat_frequency() shows how to plot the most frequent features (also by groups).