feat($injector): ability to load new modules after bootstrapping

The new method `$injector.loadNewModules(modules)` will add each of the
injectables to the injector and execute all of the config and run blocks
for each module passed to the method.

* The application developer is responsible for loading the code containing
the modules.
* Modules cannot be unloaded.
* Previously loaded modules will not be reloaded.
* Previously compiled HTML will not be affected by newly loaded directives,
filters and components.