Export knex.raw('CURRENT_TIMESTAMP') for quick usage

Maybe knex should allow people to write like this:

    table.dateTime('updated_at')
      .defaultTo(knex.CURRENT_TIMESTAMP)

    table.date('day')
      .defaultTo(knex.CURRENT_DATE)

   table.dateTime('timestamp')
      .defaultTo(Date.now)

[bennycode]

For people ending up here from Google. This is the way how to do it in 2017:

const knexfile = require('../../knexfile');
const knex = require('knex')(knexfile.development);
const date = knex.fn.now();

[squarejaw]

This seems to have been implemented by #372.

table.dateTime('updated_at').defaultTo(knex.fn.now());

Might be nice to have some better documentation for this, though.