Add pg-format to the list of helper modules.

This adds [pg-format](https://github.com/datalanche/node-pg-format) to the list of extras. It is a Node implementation of [PostgreSQL format()](http://www.postgresql.org/docs/9.3/static/functions-string.html#FUNCTIONS-STRING-FORMAT) which can be used to safely create dynamic SQL queries.

@hoegaarden I would be very happy to accept a pull request that implements the advanced features of ```format()```.