Add support for Flask shell context

[sbhtw]

flask konch should optionally use Flask's shell context (which is registered via `app.shell_context_processor(f)``). Flask shell context is available since Flask 0.11 (see http://flask.pocoo.org/docs/0.12/api/)

This allows it to use variables defined like here at cookiecutter-flask without the need for a separate (duplicated) KONCH_CONTEXT.

This PR implements this, including tests and an option KONCH_FLASK_SHELL_CONTEXT to turn it off (on by default).

[sloria]

Makes sense to me. Thanks!

[sloria]

This is released in 1.2.0.

Also, I've invited you to have commit access to this project. No pressure to do any more work than you already have; just wanted to open the door to further collaboration.

[sbhtw]

Hey, thanks for this nice offer. I really appreciate this. But this if too dangerous for now, as I have to call myself a Python rookie and I'm still making way too many beginner mistakes.

Maybe in the future. Btw, I really like your other projects, especially cookiecutter-flask, which I rely heavily upon.

[sloria]

No pressure to do anything you're uncomfortable with. You can still send PRs for review. You don't have to make commits directly to the repo. Having commit access also means that you can review and merge PRs.

Thanks for the kind words!