Support .where(sql())

This is an attempt at addressing #60 (and, originally, https://github.com/Suor/sql-bricks-postgres/issues/2).

@sandfox, @Suor: I tried doing `.where(sql(...))` and it generated `WHERE ... IS NULL`. So I stepped through the code and found that SQL-Bricks supports two WHERE APIs: `.where({key: value})` and `.where(key, value)`. Since an object wasn't passed, sql-bricks thought we were attempting the 2nd API, and since there was no 2nd argument, sql-bricks interpreted it as a null.

In 47fc522, I added support for passing a single `sql()` literal. This made my test pass.

@Suor: In your example, you have parameters being passed to `sql()` as additional arguments.... this would be consistent with the other `.where()` uses, since they allow you to pass parameters -- I probably should add that too, since as it is you have to manually add the parameter values after SQL-Bricks generates the SQL string, but before it gets sent to postgres.
What if there are other placeholders? How programmer will know whether he should use $1 or $3 or whatever?