add HHVM_DEFAULT_SCRIPT_CONFIG environment variable

- only used if running in script mode
- only used if a configuration file is not set on the command line

Fixes #1251

Test:

```
$ hhvm test.php
Extension not loaded.
$ hhvm -c load-an-extension.hdf test.php
Extension was loaded.
$ export HHVM_DEFAULT_SCRIPT_CONFIG=$(pwd)/load-an-extension.hdf
$ hhvm test.php
Extension was loaded.
$ hhvm -c /dev/null test.php
Extension not loaded.
```

test.php:
```
<?php

if (class_exists('FE_AutoloadMapGenerator')) {
  print "Extension was loaded.\n";
} else {
  print "Extension not loaded.\n";
}
```