Export stack when run as script

[garyd203]

Have some magic convenience functionality that allows you to execute a python file as a module, and it will automatically export the Stack it contains to stdout as yaml. eg.

python mystack.py > mystack.yaml

Some guidelines for expected behaviour:

• module contains exactly one Stack instance, assigned to a global variable
• module does not contain some other script-like executable code

Some possible implementation ideas:

• When you instantiate a stack, it checks whether it's locals() are the same as it's globals(), and if it's __name__ is __main__, then injects some code into globals to execute itself.