add 'after' parameter to .join() about gesundheit

[kamholz]

I suppose it might be more logical to have both after and before, and:

1. If before is null, the joined table goes at the beginning.
2. If after is null, the joined table goes at the end like usual.

I'm not sure what should happen if before refers to a non-existent table alias. Ideally it should do something sensible rather than throwing an error.

[kamholz]

I've been thinking about this some more, and I'm still a bit undecided on whether it makes sense to add it as a feature, but leaning towards unnecessary. I managed to mostly refactor my code to improve the join ordering, which wasn't quite as bad as I feared. I suppose that should be possible in the general case, too, but I'm not really sure.