Support plugin config files / command line args

Placing a <pluginname>.json file in the configdir allows you to
run each plugin with an argument set, also allowing you to run
the plugin multiple times.  The format for the .json file is:

{
  "args1": ["command", "line", "params"],
  "args2": ["more", "params"]
}

These argument list names are then appeneded to your plugin name
when returned, i.e.:

{
  "foo`args1": {
    "name": {
      _type: "n",
      _value: "value"
    }
  }
}