Tweaks to clean_names documentation and maybe tests

[sfirke]

Picking up from here, I want to make sure that users become aware of the great new features of clean_names from #340.

"I could see adding a test each for the issues being closed like #268 , #271 , #283 etc. and for the ones that are new features, like #271 and #283 , add that functionality to the examples of clean_names and/or make_clean_names and describe in NEWS. So people know about and use the new powers! " - me

[sfirke]

Annoying to do but I should update the dirty_data image and spreadsheet in the main README to have mixed date types, then use the new convert_to_date to zap them!

[sfirke]

Timing: I did a little bit of this in the clean_names and make_clean_names documentation. The stuff on the README should still be done but I think can be done after CRAN submission b/c I still control the GitHub README and pkgdown site and in my experience people go there for examples and how-to.