move all config logic to a new class
rename is_absolute_path() to is_path_absolute()

it emphasises the assumption that the parameter is a path and not some
random string
move parse_args() to Configurator and set WP path only if it's not explicitly set
merge split_special() into parse_args()

this gives us two clear variables to work with: $runtime_config and $local_config
support passing --require multiple times
fix boolean flag handling
mention that --require can be used more than once
Merge pull request #514 from wp-cli/multiple-require

Allow passing --require multiple times