Flxcolor fromString

Parses hex-html format colors and the static colors names from FlxColor.
```
Examples (input -> output in hex):
* 0x00FF00 -> 0xFF00FF00
* 0xAA4578C2 -> 0xAA4578C2
* #0000FF -> 0xFF0000FF
* #3F000011 -> 0x3F000011
* GRAY -> 0xFF808080
* blue -> 0xFF0000FF
```
Not sure this should be capitalized, since you can modify it - it's not "constant". Also, why not use a `Map<String, FlxColor>` here?