Add support for MySQL’s LIMIT with optional offset argument.

MySQL supports a LIMIT statement with an optional offset argument, i.e. `LIMIT 10, 20` which stands for "fetch 20 records after skipping the first 10".

Noticed this while working on #18. I didn’t want to couple these changes, since I’m not sure if you’re interested in supporting such extensions of SQL standard. If you are, I can help with merging these.