walk up directory tree to find config file

Allows WP-CLI to be invoked from anywhere within a project.
You are not limited to being in the directory where the wp-cli*.yml file is.
The DOCUMENT_ROOT is then set to the supplied path relative to the config file.
supply a default to the path config based on an upward search for wp-load.php

Add a Utils function for find_file_upward
Merge branch 'x-team/config-path-walk' into wp-cli/test-config

closes #362