Added a global CachedGraphics persistence variable.

If specified, CachedGraphics will use the persist value supplied by the
constructor. Otherwise, it defaults to the value of
persistentCachedGraphics in FlxG.bitmap (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.
Better placement/wording for the HaxeFlixel style.
Merge pull request #1241 from LaughingLeader/merging

Added a global CachedGraphics persistence variable.