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
prevent absolutizing a relative config path when it is empty

The result was that the config path was erroneously being set to the root /
This specifically happened when downloading WordPress via `wp core download`

See comment by @scribu: https://github.com/wp-cli/wp-cli/pull/362#issuecomment-15480546
handle edge case finding wp-cli.yml in nested subdirectory installs

See comment by @scribu: https://github.com/wp-cli/wp-cli/pull/362#issuecomment-15506011
Merge pull request #362 from x-team/config-path-walk

walk up directory tree to find config file