Add "--buildpack" option to "deis create"

Allows setting the buildpack URL when creating a new app.
Fixes #1813

The internal code of `config_set` was moved to another method so that it could be called directly to set the buildpack URL in the configuration after the app had been created.

I'm still trying to get my environment set up to properly run the integration tests, but this is the result of the new option:

```
$ deis create -b https://github.com/dpiddy/heroku-buildpack-ruby-minimal
Creating application... done, created orange-overalls
Creating config... done, v2

=== orange-overalls
BUILDPACK_URL: https://github.com/dpiddy/heroku-buildpack-ruby-minimal
Git remote deis added
```