Config->get() - Added default return argument ($key, $default)

Similar to in Bukkit, where you can get config values and if it doesn't exist, it returns a preferred default value i.e

```$var = $plugin->getConfig()->get("a.node.here",10);```
and if "a.node.here" doesnt exist in the config, it will return "10". saves having to do this for each node:

```$var = $plugin->getConfig()->get("a.node.here");
$var = ($var === false) ? 10 : $var;```

Haven't tested, but it shouldn't break anything?