load info from existing feedstock for defaults

[sbillinge]

This is a long shot, but it would be super-helpful if the defaults given could be taken from an existing feedstock if it s provided.

I am mostly using this to try and release existing projects and it is awkward and error-prone to load the information separately each time.

[bobleesj]

For this, perhaps the user can simply copy another cookiecutter.json from another project?

[sbillinge]

let's push off to the next release