Add support for config templates

[jansol]

Especially in organizations, sysadmins will want to have everyone on the internal IRC network with settings according to some internal policy. Ideally this could be done by placing config templates in /etc/weechat, to be loaded when starting weechat with an empty config dir.

As nice side effect, this could be used to handle #774 via the default config.

[flashcode]

Old savannah task: http://savannah.nongnu.org/task/?5630