Implement cols_relabel_with() (Follow-up to #1151)

[christopherkenny]

Summary

• Implements cols_relabel_with() to allow for labelling columns by function. This has a similar relationship with cols_label() as rename() has to rename_with() in dplyr.

In reviewing this, particular attention to the roxygen conventions (I am not familiar with the collate use in my own work) and what types of additional tests might be useful would be helpful!

Related GitHub Issues and PRs

• Ref: Feature Request: cols_label_with() relabeling by function #1151 (main issue), Feature Request: str_replace(), str_replace_all() like renaming within cols_label() to apply across as many columns as applicable #701, add a .fn parameter to cols_label() #817, Add md() and html() features to list argument in cols_label() #1146

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.

[rich-iannone]

Looks great! Thanks for your help here.