Add option to bind DECIMAL/NUMERIC to strings

[kadler]

Currently, we convert fixed-precision numeric types to Javascript Number objects, which are inexact double precision floating point values. This conversion is a lossy conversion, which is not ideal. We should provide a mechanism to allow the user to bind these to strings so that they can be handled without conversion issues (eg. by using a package which handles fixed-point decimal math).

This is what pg@v2 does by default and mysql does when bigNumberStrings option is enabled.