Added a global CachedGraphics persistence variable.

### Summary
If specified, `CachedGraphics` will use the `Persist:Null<Bool>` value supplied by the
constructor. Otherwise, it defaults to the value of
`persistentCachedGraphics` in `FlxG.bitmap` (a.k.a. `BitmapFrontEnd`).

This allows globally setting what the persist value of `CachedGraphics`
should default to, making it easier to keep desired graphics persistent between state
changes.

#### Changes
1. Persist in `CachedGraphics`'s constructor becomes a `Null<Bool>` variable, so that if `Persist` is null, `persist` in the `CachedGraphics` is set to the default value (the value of `FlxG.bitmap.persistentCachedGraphics`).
2. `BitmapFrontEnd` gets a simple `persistentCachedGraphics:Bool` so that persistence with all created `CachedGraphics` can be globally toggled.

#### Reasoning
1. Simply put, if one wishes to keep certain graphics persistent between state changes (`FlxSprite`, `FlxText` and so on), they have to manually adjust the `cachedGraphics.persist` variable for each object. This can be made easier with a global variable.

2. Rather than risk missing some objects, it's *considerably* easier to just set `FlxG.bitmap.persistentCachedGraphics = true`, create the objects that you wish to be persistent, and then `FlxG.bitmap.persistentCachedGraphics = false`.

3. Persistence is handy for objects that will be used in `Menu` and `Play` states of a game. My own project has a Leaderboard menu that generates quite a bit of names and entries. Only having to create these objects once is a *significant* advantage, since this menu is accessible from most areas of the game. Having an easy way to control the persistence of objects as a whole is extremely handy for this.
Plus, I'm not entirely sure what the value would be in having absolutely everything persistent or not. Not on the global scale of things, anyway. 

Maybe a renaming of the variable would make it more clear on what it does? Something like `defaultPersitence`? `defaultCachedGraphicsPersistence`?