Added new array helper to simple-pg-db

I added a new arrays helper to simple-pg-db which allows to convert
arrays in SQL List to be able to use the value returned as a single
parameter for operator as 'IN'.

I added a unit test for this new helper.

NOTE: I haven't pumped the `package.json` version, which I guess that it should be `0.5.1` and you can do whenever you update the package in npm