Expose `hcat!` to API

[pdeffebach]

Would be nice to have hcat! exposed. I think its useful, especially since the only reason to use hcat is for performance in the first place.

[bkamins]

Given hcat has copycols kwarg how useful do you think it would really be? (in general I am not opposed to exposing hcat! but just would like to get a clearer perspective of its usefulness)

[pdeffebach]

I am generally in favor of adding more ! arguments because some times you don't want to think of a new name, or don't want to do everything in a pipe.

[ExpandingMan]

copycols is fine but it's somewhat confusing because it breaks with the usual conventions followed by Base and DataFrames itself. Typically changes to mutable containers are done with ! functions and do not require re-binding a variable name.

The only concern I have with exporting hcat! is use of the namespace. There could be some other container type that exports it. At the very least, it should be added to DataAPI. Alternatively, it might be nice to document it but not export it.