small changes in the manpage description

outsourced create_barplot_df to visualization_functions.R;
Extended the flexibility with "...";
added conversion of character values to factorial;
redone the reactive method

added newline and small changes

Changed the Rmd file. One dataframe is created via a function. The input depends on the if-clauses;
barplot.R with changed validatation stucture was created;
added reexport of magrittr pipes