Support multiple aggregate functions

[luigy]

Fixes #108 and it no longer sets the output name to 'aggregate' by default.

before

  // select count(`id`) as aggregate from `accounts`
  knex('accounts').count('id')

now

  // select count(`id`) from `accounts`
  knex('accounts').count('id')

  // select count(`id`) as `aggregate` from `accounts`
  knex('accounts').count('id as aggregate')

[tgriesser]

Great work, thanks so much @luigy! I'll look to get this rolled into a new version release by the end of the weekend!