Allow other libraries to react to the --help flag, too

Allow other libraries to react to the --help flag, too

When finding --help on the command line, simgrid usually stops right
after displaying its help message but some libraries using SimGrid
want to display their own help message before the end of the world.

Now, they can use the little sg_config_continue_after_help() function
to have SimGrid not stopping after dealing with the --help flag.

(fix #93)