additional options for pivot_wider and pivot_longer

[rdboyes]

As implemented, @pivot_wider and @pivot_longer only support a small number of the options which are supported by their tidyverse counterparts pivot_longer and pivot_wider. The additional options are ones that I personally rarely use and/or don't have clear analogs in stack() and unstack(). I'm interested to hear if there are specific options that others get a lot of use out of that should be prioritized.

[kdpsingh]

As a general philosophy, we shouldn't go out of our way to support functionality that isn't directly built into DataFrames.jl or is trivial to add on top. If something is missing in that package, I would recommend we file an issue there.

The main functionality I'd like to see is the ability to specify more than one selection separated by commas for pivot_longer().

[kdpsingh]

No urgency on this @rdboyes, but I just want to figure out if we should close this issue.

Should we work on adding the ability to select multiple columns separated by commas? Anything else from your end that we should prioritize here?

Just want to make sure if no changes planned, that we close this. But I do think there's room for that one tiny improvement before closing. Happy to help with implementation if you'd like me to take a look.