GranitePluginManager: Fix loading plugins

Also, string formatting was a bit wrong (slf4j formatting uses {} as anchors and not %s, %d, etc.)

Currently:

    [Granite Startup] INFO Granite - Loading jarfile plugins/%s
    Exception in thread "Granite Startup" [11:49:41] [Granite Startup/INFO]: [STDERR]@.(Throwable.java:748): java.lang.NullPointerException
    [11:49:41] [Granite Startup/INFO]: [STDERR]@.(Throwable.java:748): 	at org.granitepowered.granite.impl.plugin.GranitePluginManager.loadPlugins(GranitePluginManager.java:85)
    [11:49:41] [Granite Startup/INFO]: [STDERR]@.(Throwable.java:748): 	at org.granitepowered.granite.GraniteStartupThread.run(GraniteStartupThread.java:90)

After this commit:

    [Granite Startup] INFO Granite - Loading jarfile plugins/LWC-Sponge-0.0.1-SNAPSHOT.jar
    [Granite Startup] INFO Granite - Loaded LWC (0.0.1-SNAPSHOT)!

I wasn't sure what to do with `Collection<PluginContainer> plugins` because it wasn't ever being initialized. I just initialized it to an `ArrayList`.