Add a method to insert a column to last index

[pdeffebach]

I've been experimenting with piping via Lazy's @> macro. And the new funcitonality.

It all works super well! However one feature that would be nice is an insertcols! method that doesn't take an index for position. The current syntax to insert a column with a particular name at the end of the dataframe -- with a function call where df is the first argument -- is

y = rand(nrow(df))
transform(df, 1 => (x -> y) => :y)

Which is kind of a pain.

[bkamins]

insertcols!(df, :y => y) works on master.

[pdeffebach]

great! I thought I was on master but I guess not!

[bkamins]

OK - but please check (there is always a risk we have a bug)