Switch cols to kwarg from positional args

[bkamins]

Following the comment by @nalimilan in #2020 after seeing what changes I make in #1958 currently I came to the conclusion that we should cleanup cols to be a keyword argument everywhere.

This should be changed prior to 1.0 release, but is non-blocking for 0.20 I think. I will make a PR for this.

[bkamins]

@nalimilan Here is the result of cols usage review.

cols is a positional argument in:

• sort and issorted (actually we have a reverse deprecation to what we discuss in these cases)
• allowmissing
• disallowmissing
• categorical
• completecases
• dropmissing
• nonunique
• unique
• disallowmissing
• grouping
• getindex
• view
• DataFrameRow and SubDataFrame constructors

cols is a kwarg in:

• describe is kwarg
• vcat is kwarg but has a broader meaning
• push! and append! has a different meaning

Given this I recommend to leave things "as is", as we would change a bunch of stuff to make a rarely used functionality consistent.
Only describe is really conflicting with what we do currently (cols as positional arg).
In describe it is natural to have cols as kwarg as there is no other option, so there should be no confusion.
vcat, push! and append! are different anyway and also I think there should be no confusion.

What is your opinion? I would just close this issue.

[nalimilan]

OK, thanks for checking. Let's leave this as-is then.