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. 

this kills the program, where we just want to issue a warning I think