Suggestion: label rows&columns of the "Plyr package" tables

[crtahlin]

The tables under the "The plyr package" heading in Functionals.Rmd do not have labels in the sense of whether the row (or columns) represent the input (or output).

The text would be clearer if, for example, the top left cell of the table would contain the text:
"Output

Input"

Or something even clearer, as this could be misinterpreted I guess ...

[hadley]

Unfortunately I can't do that easily in markdown, but I'll try in the final book.