Make <ios linker-flags ..> cumulative across haxelibs and project

This commit allows multiple <ios linker-flags> in an include.xml file.  It also will accumulate the linker flags from haxelibs as well rather than just picking the latest one.

=== Use Case ===
I have been creating haxelibs for ad networks.  Most of these networks require adding linker flags for things like -ObjC or -force_load <library>.  

Due to this I do not want require clients of the haxelib to add these linker flags themselves.  The haxelib should be selfcontained.

In order to accomplish this, openfl needs to allow concatenating all of the ios linkerflags in included haxelibs.  Allowing multiple linker flags in the same file is for convenience so one can split up large declarations.