Automated site deployment 

Might be worth moving the site deployment documentation into the wiki(?) This also needs to be thoroughly tested before introducing as standard.

With the introduction of bigfork.json for config, any data added into an 'extras' object in bigfork.json will be parsed to package.json [line 32 Install.php](https://github.com/feejin/Silverstripe-CleanInstall/blob/ef9cac301b50f5f84abd86510919812174c6891c/util/Installer/Install.php#L32) + [line 103 Install.php](https://github.com/feejin/Silverstripe-CleanInstall/blob/ef9cac301b50f5f84abd86510919812174c6891c/util/Installer/Install.php#L103).