Support multiple aggregate funcions

Fixes #108 and it no longer sets the output name to 'aggregate' by default.

before
```js
  // select count(`id`) as aggregate from `accounts`
  knex('accounts').count('id')
```
now
```js
  // select count(`id`) from `accounts`
  knex('accounts').count('id')

  // select count(`id`) as `aggregate` from `accounts`
  knex('accounts').count('id as aggregate')

```