Add configuration use helpers

[tokee]

A common pattern is to have something like

public static final String SERVICE_PORT_KEY = ".network.port";
public static final int SERVICE_PORT_DEFAULT = 8090;

public static final String PURPOSE_KEY = ".purpose"; // No default

private final int servicePort;
private final String purpose;

public MyApp {
  YAML conf = getMajorConfiguration().getSubMap(".myapp");

  servicePort = conf.getInt(SERVICE_PORT_KEY, SERVICE_PORT_DEFAULT);
  purpose = conf.getString(PURPOSE_KEY); // throws exception if the param is not defined
}

Having the keys and the defaults defined as static final at the top of the file is great when reading the code, but maybe they could be collapsed to 1 line each and maybe it could be externalized when there is a problem with the configuration? Something like

public static final KeyPool KP = new KeyPool();

public static final Key SERVICE_PORT = KP.add(".network.port", 8090),
public static final Key PURPOSE = KP.addString("purpose")

public MyApp {
  // If the value for PURPOSE is not defined, an exception is thrown
  KP.setConf(getMajorConfiguration().getSubMap(".myapp"));

  // After setconf on KP, the values can be accessed with 
  // SERVICE_PORT.get()
  // PURPOSE.get();
}