Configurable permissions for irc.conf and other files (such as group ownership)

[zwimer]

Feature description

When starting weechat (I use the official alpine docker container version) it chmod's some files as 600; that is only the user has read/write permissions; even if umask is 2. It'd be nice if we could configure this; or at least allow group ownership (i.e. chmod 660). That way we could allow members of the 'weechat' user-group to edit these files rather than just the 'weechat'.

[flashcode]

Done, new option: weechat.look.config_permissions, for example:

/set weechat.look.config_permissions "660"