[suggestion] Support for WHERE on JOIN

I couldn't find a way to filter the table I was joining, so I added these five lines of code to do that if you pass more options to the JOIN.

My PHP might not be very good though.
I'll explain it. `is_string(key(array_slice($relation, 1, 1, true)))` does the following: It checks if there's anything at the [1] position in the array, and slices it off. `key()` pulls off the key and checks it it `is_string()` (which it has to be in order for it to be useful in a WHERE clause).

Also, it has to start with `AND`, so I just remove the start of the output with `substr` and replace it.

I'm sure you can do this in a prettier way, but it fixed a problem I was having, at least.