Added load_asgi_app method to CLI class

[makondratev]

In compiled projects(using PyInstaller for example) application must be imported during compilation. So you can redefine only this method (like gunicorn's load_wsgiapp) instead of copying whole run() method and rewriting only one line. Hope this will help everyone who compile python projects with daphne server.

[makondratev]

Thanks a lot! That would be great!