Allow custom config.hdf file, cleanup, take #2

Did a bit of cleanup and refactored so that we can now have a custom config file, with a fallback to the default one in the `conf` folder.

- moved functions to `common`
- moved HHVM_VERSION to it's own file. In time we'll probably have a few of these hard-coded values, I don't think they make sense in the main compile script

Tested by creating a simple project with two files, `index.php` and `index2.php`, echoing different texts. Pushed to Heroku with the default config, confirmed that it displayed `index.php` content and then added a `config.hdf` that makes `DefaultDocument` be `index2.php`, pushed and confirmed it displayed contents for `index2.php`.