[request] allow multi --cmd argument on commandline

Hi Carl,

Here a small enhancement on commandline processing. The patch would allow to pass --cmd multi-times on vimb commandline.

Having only the ability to use --cmd="set scripts=on|set plugins=on" don't permit me to use vimb in shell scripts, without rewrite argument processing in shell, or dynamically generate the config file.

For example, I have several "profiles" managed by configfile or adding --cmd.
 - vimb-empty : that create a completely new configuration directory (by redefining $XDG_CONFIG_HOME envirnoment variable) with some parameters configured with --cmd (scripts=on, plugins=off, ...)
 - vimb-tor : that call vimb-empty with additionals parameters (ssl-strict=on, scripts=off)

The implementation convert vb.config.autocmd from char* to GList.

I have also try to follow coding style this time (indentation with spaces).

Thanks.