Concat: Add `QUICKSTART_DISABLE_CONCAT` switch to turn off concat.

This will let you turn off concat, like:

```
define( 'QUICKSTART_DISABLE_CONCAT', true );
```

See #254
Add cron to update WordPress and VIP Plugins repos

We can automatically `svn up` the WordPress and VIP Plugins repos
every 30 minutes to make sure they're always up-to-date.

Fixes #256