allow -D dump=pretty

git-svn-id: http://haxe.googlecode.com/svn/trunk@5821 f16182fa-f095-11de-8f43-4547254af6c6
--dce is now -dce (fixed issue #1331)

git-svn-id: http://haxe.googlecode.com/svn/trunk@5822 f16182fa-f095-11de-8f43-4547254af6c6