Clean up and improve command line handling
Clean up and improve command line handling

This fixes fixes a crash when FuseSoC was launched without arguments
and finally makes it possible to get information about plusargs by
specifying --help after the name of the core for FuseSoC simulations
(e.g. fusesoc sim wb_intercon --help)

lowRISC/fusesoc,olofk/fusesoc,olofk/fusesoc,lowRISC/fusesoc