Support for global `wp-cli.yml` and `wp-cli.local.yml` configs
Introduce WP_CLI_CONFIG_PATH environment variable.

It allows defining an additional config file, which gets loaded before
any project config file.

Also, remove the --config= global parameter, which becomes redundant.

see #698
Merge branch 'master' into global-config-2
add back deprecated --config global parameter, for backwards compatibility
show warning when deprecated global parameter is used
initialized configurator on demand
properly merge keys that can contain multiple values
fix case where a file-only config key would make a runtime arg of the
same name disappear
global flags have 'runtime' key '', which is falsy
separate YAML parsing from config merging
arrayify require: key
behat: replace --config with WP_CLI_CONFIG_PATH
Merge pull request #698 from wp-cli/global-config-2

Support for global configs; fixes #445