allow specifying which columns to merge on

[yakir12]

When merging two tables,

t1 = table((x = [1,2], y = [3,4], t = [5,6]), pkey = [1])
t2 = table((z = [1,2], y = [3,4], t = [7,8]), pkey = [1])

it would be easier to:

merge(t1, t2, on = (:y, :t))

than what we need to do to accomplish the same thing today:

merge(select(t1, (:y, :t)), select(t2, (:y, :t)))

The exact semantics is immaterial, but since a merge requires the same columns in both tables, it should be necessary to specify the selected columns only once (and not twice).

[shashi]

on is probably too overloaded by how SQL uses it in JOINs. I think this keyword arg should be called... select :-)