`use_anova_cols()` and `use_summary_cols()`

[aphalo]

The equivalents of use_label() for stat_fit_tb() to be used for selecting and renaming columns. (Language translations of colum headers could be also implemented at a later time.)

[aphalo]

It us rather easy to achieve this in users' code, and examples exist in the documentation. For the time being I set this issue on hold.