Support providing a custom root

[karoun]

Our project stores PHP dependencies in vendor/composer (since we share vendor/ between JavaScript and PHP). Better handling for custom roots was introduced because of #10, but doubly-nested vendor directories still aren't supported because the root is assumed to be the parent directory of vendor-dir:

hhvm-autoload/src/ComposerPlugin.php

Lines 37 to 40 in 2808fe0

	$vendor = $composer->getConfig()->get('vendor-dir', '/');
	$this->vendor = $vendor;
	$this->root = dirname($vendor);

Should a config key be introduced to fix this, or would re-purposing relativeAutoloadRoot be sufficient?

[facebookxx]

a