[Extensions] Add DidCreateModuleSystem hook

Added an interface to be implement by the users of
XWalkExtensionRendererController, with a function that is called
everytime we create a XWalkModuleSystem. This allows the users of
extensions to add their own (native) modules to that module system.

This need was prompted by pull request #612.
Merge pull request #886 from cmarcelo/hook-for-registering-native-modules

[Extensions] Add DidCreateModuleSystem hook