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.
contributing: add line about getting feedback
supply a default to the path config based on an upward search for wp-load.php

Add a Utils function for find_file_upward
fix brace style in Feature Context
move define_custom_wp_content_dir() into step definition, since it's only used once
use @BeforeSuite hook to initialize $additional_args
change directory, instead of prepending --path
make FeatureContext->get_path() return the path unmodified
prepend './' to relative paths
pass cwd directly to proc_open() and revert changes to get_path()
split single site from multisite install steps
add 'cwd' key to FeatureContext->_run() result
add basic scenarios for wp-cli.yml
add scenario for WP in a subdir
Added behat feature for media regenerate
'a custom wp-content directory'
add run cmd from subdir step definition
fix comment about param-dump

[ci skip]
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
deb-build: add sudo where appropriate

see #368