Support $xdg_config_home for user's main config file (~/.wp-cli/config.yml)

[ZuBB]

Basically would be nice if wp-cli check also $xdg_config_home dir for its config file (in default settings it will be ~/.config/wp-cli/config.yml).

Motivation: wp-cli throws ANSI esc sequences in the terminal. so I need to set no-color globally. But I also like my ~/ to be uncluttered.

Some refs

• Bold (and possibly color) escape codes are not honored under FreeBSD. #5833 (comment)
• Save cache to $XDG_CACHE_HOME instead of ~/.wp-cli/cache #1747