Changed name of set_text_color() to choose_text_color() to avoid confusion with the global option setter functions, e.g. set_text_size(). Updated tests to reflect modified threshold.

Adds a few additional dplyr functions to the package imports, in particular: slice, data_frame, and row_number.

this makes it easier to differentiate between the join-related and the (coming) tidyr-related functions that have similar goals