Add client warning when running command without -a supplied

Issue: https://github.com/deis/deis/issues/1846

Adds a check for the --app parameter.

I haven't written any python in over a decade, so feedback welcome.

How does the client know it is inside an 'app folder'? 
I can't see any reference apart from the --app param. 

not 100% sure about this since I'm not in a terminal, but I think `flake8` will complain here about there not being enough spaces :)