Fix issue 206

The interactive debug plugin is usable. Even though some code comments and docs are missing, I created this pull request to receive some feedback about the code and the plugin usability. I want to write some more docs before this gets merged.

If you are going to test it, here's a mini howto:

* The plugin comes disabled by default. Enable it by calling `FlxG.addPlugin(new InteractiveDebug());` during `create()` of any game state;
* The same key that opens Flixel's debug overlay will open the plugin tools too. Make sure `FlxG.debug` is `true`.
* Use the `pointer` tool to select things (hold the `Ctrl` key to select more than one element at time). Selected elements can be moved around using the `move` tool or using the `pointer` tool while holding `Shift`. Selected elements can be `kille()`d by clicking the `eraser` tool or by pressing the `Delete` key (`Shit+delete` will remove the selected elements from Flixel rendering list);

I've made a few tests using the game Mode, by Adam.

~~I don't know why, but the commit log shows more commits than I added myself. There are commits by @IQAndreas, for instance~~ that's because I created a pull request against master. I will change that before merging.

Fix: https://github.com/FlixelCommunity/flixel/issues/206