(Provide an Option to) return BIGINT as String or BIGINT

[markddrake]

Currently BIGINT columns are returned as JavaScript numbers. This is problematic, since there the JavaScript Number type cannot accurately represent the entire range of values supported by BIGINT. Ideally IBM_DB should return these columns as instances of the Node.js BIGINT class or as strings. Since this would be a 'breaking' change, it should be possible to configure how columns of BIGINT are returned, at the statement and connection level.