Add `insert!`

[pdeffebach]

Now that we have coalesced around a DataFrame being a collection of rows, other than a few notable exceptions, we should add an insert! method

insert!(df, row)

where row matches anything that can be push!ed to a DataFrame.

[pdeffebach]

Context: I'm using xtable in R to make a data-frame that can be outputted to a publication-quality table using dplyr. So I have to be very particular about the order of rows etc. This seems hard to do in R and maybe we can make peoples' lives easier in Julia.

[bkamins]

Yes it is not a problem to add it. There is a bunch of similar functions waiting to be added. But we treat this as non critical for 1.0 release. Of course is someone wants to make a PR it is welcome.

[CameronBieganek]

insert!(df, row)

Shouldn't that be the following?

insert!(df, i, row)

Otherwise it would be the same as push!.

[bkamins]

x-ref #2936

[bkamins]

added