Source /etc/vimrc.local if it exists
    
    There seems to be no good way to extend distro-provided vimrc. If the
    user modifies /etc/vimrc, then RPM will keep it intact from then on,
    with subsequent distribution updates going into a .rpmnew file. Sad.
    
    On Debian the user is supposed to put the local overrides into a
    vimrc.local file next to the main vimrc. That is pretty neat -- the
    local configuration is separated from the system one and can be updated
    independently. Let's do the same thing.