Add formatting to FlxText

This pull tries to add basic formatting to FlxText:
- Font color.
- Bold (if the font is capable of)
- Italics (if the font is capable of)
- Border color (if there is a border to begin with)

You can change other parameters like font, font size, etc. but in some targets (cpp/neko) those parameters aren't fully functional yet.

Example using FlxTypeText:

```haxe
var text:FlxTypeText = new FlxTypeText(0, 0, FlxG.width, "Hello HaxeFlixel Community!");
text.setFormat(null, 24, 0x000000, "center");
text.setBorderStyle(FlxText.BORDER_OUTLINE_FAST, 0xFFFFFF, 2);
text.addFormat(new FlxTextFormat(0xE6E600, false, false, 0xFF8000, 6, 8));
text.addFormat(new FlxTextFormat(0xFFAE5E, false, false, 0xFF8000, 8, 10));
text.addFormat(new FlxTextFormat(0x008040, false, false, null, 10, 12));
text.addFormat(new FlxTextFormat(0x0080C0, false, false, null, 12, 14));
text.addFormat(new FlxTextFormat(0x00E6E6, false, false, null, 14, 16));
text.addFormat(new FlxTextFormat(0x0080FF, false, false, 0xFFFFFF, 16, 100));
text.start();
add(text);
```

Don't merge it yet. It needs some testing and I would want to hear what you people think about the changes first.