Add a convenient syntactic suger for joining using multiple columns

[Ofri Kleinfeld]

The current syntax for joining using multiple columns is something like that:
some_query.join(some_other_query).on(some_col).on(some_other_col)

The desired syntax should be more natural. For example, something like:
some_query.join(some_other_query).on([some_col, some_other_col])