@join on columns of the same name

It would be neat if it wasn't necessary to repeat the name of the columns on which you are joining, when they are the same:
```
@join(foo, bar, _.joincolumn, _.joincolumn, {_.other, __.also})
```
Right now the macro seems to just go by the number of arguments, so omitting one tries to create a function for `foo |> @join(bar, ...` use, but perhaps it could look at what it gets before deciding? 