Improve node server commands

[elisee]

With the recently added node server install $SYSTEM_ID command, we can now install the latest release of a system quite easily from the command line. Also node server init $SYSTEM_ID is a new command for generating a system skeleton for developers. Here are some improvements we should look at:

• Use yargs for parsing arguments in server/index.ts as in server/paths.ts (and share the parser)
• Double-check that the server isn't running before doing anything on it (lockfile!)
• Install a system from sources: node server install --source $SYSTEM_ID. This should clone the GitHub repository, init submodules and build the system
• Install a particular plugin: node server install $SYSTEM_ID:$PLUGIN_AUTHOR/$PLUGIN_NAME
• Add support for generating plugin skeleton with node server init $SYSTEM_ID:$PLUGIN_AUTHOR/$PLUGIN_NAME
• Allow adding the common plugins submodule with --add-common maybe?