Add predicate to join function

[pauldix]

Currently, join works on equality for the given columns. Users should be able to specify an arbitrary predicate to join on rather than columns. The columns and predicate arguments are mutually exclusive. It would look something like this:

join(
  tables: {foo:foo, bar:bar},
  predicate: (t) => t.foo.col1 > t.bar.col2,
  fn: (t) => {"foo.value": t.foo._value, "bar.value": t.bar._value}
)