[R-Forge #5352] "promote" roll to a vector

[arunsrinivasan]

Submitted by: Eduard Antonyan; Assigned to: Nobody; R-Forge link

That way one can specify how to join (roll, no roll, etc) for each of the columns being joined.

For reference, current behavior is to do a rolling join on the last column and a strict one on all of the others and this FR is to add the flexibility to the other columns.