Add additional set operations

[michaelmior]

This provides a partial implementation for #18. Basically almost the same grammar is used but INTERSECT and EXCEPT are allowed as keywords in addition to UNION. Each of these parses into a new kind of top-level expression. This is roughly following what Postgres does since these are not all supported by MySQL.

[albin3]

@michaelmior Thanks for this pr, but I don't want to merge this one.

In my opinion, It will be easier to maintain this repo just support mysql syntax follows official document. Source code can be readable too.

On the other hand, table join syntax can touch the aim of what INTERSECT and EXCEPT do.

[michaelmior]

No problem, I understand although I don't really see this making things less readable. Of course queries can be rewritten, but this doesn't help when I need to parse the alternative syntax. That said, I can always maintain my own fork and I appreciate having this as a starting point :)