Automatically parse DB bigints as JavaScript Ints

https://trello.com/c/RIATlpAS

In the [charging-module-api](https://github.com/defra/charging-module-api) we hit an issue caused by integer overflow. We hold all our monetary values as pennies rather than pounds and pence. This is common practise in financial systems as it avoids some issues with rounding in equations.

The issue was all our monetary fields were set as `integer` but we found examples of bill runs with values larger than 2147483647 (the max an integer can hold). To solve the issue we needed to set the DB fields to `bigint`. [Bigint's](https://www.postgresql.org/docs/10/datatype-numeric.html#DATATYPE-INT) can hold values in the range of -9223372036854775808 to +9223372036854775807 in PostgreSQL.

JavaScript on the otherhand uses [Number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number) to manage integers. An integer in JavaScript has a range of -9007199254740991 to 9007199254740991. If you need to go as large as PostgreSQL you need to use a [JavaScript BigInt](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt).

Because of this discrepency the PostgreSQL driver for Node by default returns all BigInt values as strings. This is a [design decision by PostgreSQL](brianc/node-postgres#353) to avoid any loss of precision.

However, if you are confident you won't be effected by this precision loss (which we are) you can tell the PostgreSQL driver to return an integer instead.

So as we do not expect to generate bill runs with values greater than 9.0071993e+13 (I don't even know how to say what that number is!) this change adds the config needed so we can forget about BigInt's in the rest of our code.