Command line argument to set default JABAWS server

Institutions and power users with their own JABA server may wish to specify a default server as a command line argument to Jalview. This should replace the default public JABAWS url, and precede any additional URLs in the users own preferences.

Currently, this behaviour is only possible using the 'props' argument - which does not modify the user's own properties file, but instead provides a read-only property set for the current session. Instead, an argument would also allow institutions to customise Jalview installations to use in-house servers.