adds join_by

[drizk1]

I am not sure if this was the best way to do it, or if i should have removed the old syntax from the docstring, but this is one possible implementation (if join_by is not the move based on what happens with #126 i can close pr )

allows for all joins:

@inner_join(a, b, join_by(City==Location, Job==Work))
@inner_join(a, b, join_by("City"=="Location", "Job"=="Work"))
@inner_join(a, b, join_by(:City=>:Location))