Move default config location to $XDG_CONFIG_HOME/mwcli.yml

[samwilson]

The config file is currently called config.yml and is in the mwcli directory by default. The --config option (to all commands) can be used to override the location and filename.

It'd be better to have the config be by default in $XDG_CONFIG_HOME/mwcli.yml. The idea is that all sites have their config in that file, so there won't be any need for a subdirectory. And it will still be able to be overridden. dnoegel/php-xdg-base-dir could be used.

If an existing config.yml is found, the user should be asked if they want to move it.