Astropy Proposal for Enhancement 12: Using Cookiecutter for the package-template (APE 12)

This APE proposes migrating the package-template (https://github.com/astropy/package-template) to use the Cookiecutter (http://cookiecutter.readthedocs.io/) project to provide templating. This would enable users to be able to customise the package template for their package from the command line, in a much simpler manner than the current manual process. As well as this, having a templating system makes it easier for other projects to change the branding or default settings of the package template.

This APE proposes major changes to the current layout of the package-template repository, potentially breaking existing user workflows, hence the APE.