support disabling query typechecking via env var

[andrewbanchich]

The query! macro is really useful, but I can't use it when building my project as a Docker image since there is no database at that point. It would be nice to be able to disable it with an env var and only have syntax checking.

[abonander]

We're already discussing support for this kind of thing in #60. query!() requires at least knowledge of the query's result columns to compile properly so disabling the checking entirely is a non-starter.