Make elsa work with FLASK_APP

[encukou]

Flask's new api uses FLASK_APP=mysite.py flask run instead of app.run(). This solves a few problems related to importing under the debug reloader.

It would be nice if elsa supported this as well, so users could switch from plain Flask to elsa by leaving $FLASK_APP as is and just calling elsa freeze.

There would need to be an option to use a custom freezer and options (e.g. base URL). Probably these would be set in app.config. (Maybe flask/frozen flask has this already?)

Related: we should also enable people to use elsa programmatically, something like Elsa(app).freeze().